Okay. As I had mentioned before, this is the radiation heat transfer numerical problem
out of Chapter 13. It's a rectangular box. The dimensions are given. I've identified
the front surface as A3, the top as A2, the right hand side's A5, and the other surfaces
are defined there on the sheet. Give you the emissivities of all six surfaces, give you
the temperature of five surfaces and surface six is reradiating, and I've calculated the
F's for you for three different ones, F12, F13, F15. Okay. So here's what you do. The
first thing you do is verify those three F values I gave you by getting whatever you
need, X over Y, X over Z, go to graph number so and so, and show me that that number is
correct. Okay. So verify those three F values. Okay. Then, we start with the A, B, C answers.
There are five unknown J values. Okay. It says -- it says write an equation for each
of the nodes for which J is unknown. There are five nodes where J is unknown. Down below,
it says, hint, for part A, use equations 13-21 and 13-22 of the textbook. Equation -- we
had both those equations in our -- in our notes. Okay. 13-21 is the equation you -- you
use where the temperature of the node is known. Equation 13-22 is the nodal equation where
the heat flux is known at the node. So depending what's given, if the temperature node's given,
use equation 13-21. If the heat flux in the node's given, use equation 13-22. So write
those equations out. There'll be, in terms of J1 through J5, the unknowns of those five
equations will be J1 through J5. Solve those by whatever means you want -- what you did
last time for the first one maybe, maybe MatLab, maybe Excel. It's going to be pretty simple,
divide by five matrix. TI89, TI92, whatever it might be. Put on the -- on your -- on your
homework what you used to solve those equations. If you use Excel or MatLab, give me a hard
copy printout of the results -- attach it to the back of this. So give me a -- a hard
copy printout of that. Solve for the five unknown J's, then put the J's in for the equations
to get what part C to get Q4, Q35, and the temperature of the adiabatic surface, surface
six, the temperature of the adiabatic surface. Okay. So that is then due a week from today.
Any questions on that right now? Okay. Now, let's go ahead and take a look at where we
left off last time. We're in Chapter 7. Okay. Last time, we were looking at how we get H
for a flat plate. So the flat plate like this -- it might be part laminar, it might be part
turbulent. There's a critical X value where transition may occur. The Reynolds Number
for that critical transition is 500 thousand -- five times ten to the fifth. That's external
flow over the simplest possible geometry, which is a flat plate. Now we take the next
most popular geometry. Obviously, we engineers use a lot of tubes and pipes. So now we want
to know, how do we find the H value over pipes and tubes? What it amounts to is a circular
cylinder. So this is a circular cylinder. Okay. Its dimension into the blackboard is
L, so it's like a pipe -- a pipe of length L. The diameter of the pipe -- the outside
diameter is capital D, outside diameter. Now we have to go back and revisit ME312 a little
bit. Not much, just a little bit. In ME312, we analyzed the flow over a right circular
cylinder from a fluid mechanic's point of view. Here's what we kind of said. Approaching
flow stream, free stream velocity U infinity. The streamline here, the dash line ends up
at this big black dot. That's called the stagnation point at the front of the cylinder. Stagnation
point means that the velocity there is zero. Okay. Then, of course, the flow goes around
the cylinder like this. It speeds up as it goes around the cylinder. Because the velocity
is zero at the leading edge or the -- the frontal point on the cylinder, a boundary
layer builds up -- a laminar boundary layer builds up. So we have a laminar boundary layer
build up around the cylinder. It gets bigger and bigger and bigger. Eventually, it's -- what
happens is -- we call it the separation point -- it separates from the cylinder and goes
off like that. So these are the flow streamlines. And again, this is our boundary layer, it's
a laminar boundary layer. On the backside of the cylinder, it breaks off and goes like
this, and behind the cylinder is a region called the wake region. Turbulent eddies form
in the wake region like little circular patterns of velocity. You can get a really good view
of that if you go to a stream -- a fast moving stream and there's a rock in the middle of
the stream about the size of a watermelon or something. And -- and -- and the water
goes around that rock, on the backside of that rock, there'll be little circular eddies
and there'll be leaves and bugs floating back there in circles. Boy they're just going in
circles behind there. Yeah, that's the wake region behind a rock in a stream. But it happens
whether it's air or water or whatever. There's a wake region typically behind -- behind here.
It depends on the Reynolds number. If the Reynolds number is really low, okay, then
the -- this point, by the way, is called the separation point. It's where the streamline
separates. It's caused by a pressure gradient. But that was -- that was ME312, we're not
going to revisit that in detail. What happens at -- at this particular case -- low Reynolds
number, the separation point is way back on the backside of the cylinder. As the Reynolds
number increases, the separation point moves towards the front side of the cylinder, eventually
ending up about here. At that point -- at that Reynolds number, something dramatic happens
-- the flow transists to turbulent. There's a turbulent part. It's laminar for part of
the way and then it goes turbulent at -- like this, so the dash line is the turbulent part.
Just like over here, it starts out laminar and at a certain Reynolds number, it transists
to a turbulent boundary layer. The same thing around a circular cylinder. It starts off
as a laminar boundary layer. As the Reynolds number gets higher, eventually, the boundary
layer -- this is the laminar boundary layer, this is the turbulent boundary layer, and
then the separation point goes back to the backside because of momentum consideration.
The fluid particles close to the surface, when the flow goes turbulent, pick up momentum.
The momentum causes them to continue following the surface until, again, the pressure difference
builds up and they fly off. All right. That's the fluid mechanics case. But again, you can't
do the heat transfer part until you understand the fluid mechanics part. Okay. So now our
object, though, is to get H in ME415 because we want to find the heat transfer Q. And Q
is equal to H bar AS TS minus T infinity, assuming the surface is hot and the free stream
is cold. So the temperature of the surface, we call that TS. It's the same all the way
around the cylinder. TS is constant all around the cylinder. Okay. So obviously, let -- let
-- let's give -- let's give AS first. What -- what is AS? AS is the surface area. What
is AS? It's the area of the circular cylinder that is in contact with the fluid. Okay. Pi
DL, pi DL, circumference times length, pi DL. Okay. Obviously, ME415, we got to find
that guy. That's what Chapter 7's all about. Over here. What's the challenge? We got to
find these two guys. Need those two guys. Local heat flux or the heat transfer over
the surface. Okay. Over there, a lot of it was mathematical by nature. Over here, not
-- not much mathematical by nature. The way that you get this guy here is typically by
evolving what we call empirical equations. They call them empirical correlations. That
means they're derived from experimental observations. Derived from experimental observations. And
the first one we have is an -- let's call it one. By the way, the bar over H means it's
the average value. The average over what? Well, the answer is the average over the cylinder
surface. In reality, H varies from the front -- theta equals zero, theta equal 45, theta's
90 -- from the front to the back of the cylinder, H will vary. So the bar means averaged overall
angles theta. Okay. So Nusselt D bar C Reynolds DM Prandtl to the one-third. That's the empirical
correlation. Let's put these guys down. Reynolds number based on diameter D, U infinity, D
over new, Nusselt based on D, that's the bar, H bar D over K. So now, we base the Reynolds
and the Nusselt on the outside diameter, D, capital D. Flat plate, we base the Reynolds
and Nusselt on the distance X, the distance from the leading edge of the plate. Okay.
This is called the Hilpert equation. Just so we know -- Let's see, have we got it here?
I don't have the equation number down right now. But it's the first equation you come
to in that part of Chapter 7. Properties at T film, which is T surface plus T infinity,
divide by 2. The values of C and M are constants. And the values are given in Table 7-2. And
Table 7-2 looks something like this. I'll just put one down there. Let's see where Table
7-2 is here. There it is. I'll do 4000 to 40,000. There's different ranges here -- 4,000
to 40,000. I'll put another one down. One of them goes from 40 to 4000. That value,
4000 to 40,000 -- yeah -- .193 and .618. 618. Okay. So that's what you do. Again, here's
the Nusselt number, there's the H bar you want, you solve for the Nusselt number, get
the -- number one -- get the Reynolds number. Is there any magic Reynolds number? No. Over
here, a flat plate. Is there a magic Reynolds number? Yeah. Five times ten to the fifth.
Over here? No. It might be a laminar boundary layer, it might be a combination of laminar
turbulent boundary layer, but there's no magic Reynolds number that you have to always focus
on. So -- put the Reynolds number here. Get all the properties -- what properties? The
Prandtl number, kinematic viscosity, thermal conductivity, at what temperature? The film
temperature -- TF, film temperature. The boundary layer is considered to be a film, or a layer
of fluid, the film temperature. Okay. These guys came from where? Experimental observations.
Okay. There's another possible equation to use. This is two. This one is Nusselt number.
It looks very similar. Reynolds D to the M, but now there's a Prandtl to the nth power
and then a ratio Prandtl number to the one-fourth. Okay. If the Prandtl number greater than or
equal to ten -- greater than ten, N is .36. If Prandtl less than or equal to ten -- that
should be a ten -- then N is .37. Properties at T infinity, except Prandtl S at T surface.
C and M are in Table 7-4 and it's called the Zukauskas equation. Then, the Churchill equation.
Our -- our -- our textbook gives three different equations to find H bar. Most books give you
one. But our -- our textbook -- our author is really complete, so he gives you three
possibilities. This equation -- I'll just put a few things down. Properties at T film,
no table needed. Which is the good part of it, you don't need a table. In all these guys,
you've got to satisfy some Prandtl number restrictions. So you check the restrictions.
The Prandtl number might -- should be greater than something or the Reynolds number should
be greater than something, so you check the restrictions. On this guy right here, there
are no important restrictions. I think the Prandtl greater than .6 is the only one. So
-- but you -- you always check -- always check and see if there are restrictions on these
equations. How did someone get these equations? Because sometimes it's like magic, like, okay.
I'll use it, but I really don't know where it came from. I'm just a user. I -- I don't
worry about where it came from. But some people worry about where it comes from. Like, how'd
that guy get that table right there? Okay. Well, here's one way that -- that -- that
you can do that. If you -- I'm going to -- I'll tell you. It's example 7-3 -- example 7-3.
There's a wind tunnel, and they put in the wind tunnel a circular cylinder. Diameter
of that cylinder is D, it's length is L, into the blackboard -- sticks out blackboard -- L,
and the wind tunnel creates a nice even flow like this. Several years ago, I had a student
in class and after class was over, she came by my office and said, Professor Biddle, I'm
doing a senior -- I'm having a senior project. I was looking at that example 7-3 in the textbook,
she said, and is it possible I can do my senior project based on that example? And I said,
well, yeah. I think you can try. Yeah. Go ahead and give it a try. So what she did -- we
-- we have a small wind tunnel in the fluids lab. It's really belongs to technology department,
but we -- we use -- at -- at that time, we used it. And it was only about, I think, 12
inches this way -- out of the blackboard this way, 12 inches, and maybe this way 10 inches.
And so she was going to use that wind tunnel. And she got a aluminum cylinder, and I think
it was like, it was 12 inches long. Must have been -- yeah, four inches in diameter. Twelve
inches long -- piece of aluminum -- four inches in diameter, solid aluminum. Okay. She drilled
a hole through the aluminum -- axial hole -- through the aluminum, and then she stuck
this cartridge heater -- electric cartridge heater inside of the hole and made a really
tight fit and put some special heater transfer enhancer in there. So this cylinder's in here.
And then you attach this thing to a power supply, a watt meter -- put a watt meter in
the circuit and this is the input power -- in watts of course. Okay. So you heat this cylinder
that way. Solid aluminum, drill a hole through it, insert the cartridge heater, and then
attach it to a power supply and put a watt meter in line. Then -- well, here's what she
needed to do. She needed to get the Reynolds number -- so Reynolds number based on diameter,
U infinity D over new. Okay. Got the diameter, four inches. I'll get the properties later.
U infinity -- go back to ME313 lab -- fluids lab. We have an HVAC duct in that lab. We
have students measure the velocity of the air in that HVAC duct in that lab to get a
velocity profile in that rectangular duct. Very similar. So what did she do? She took
the pitot-static tube that we used in the 313 lab and she put the pitot-static tube
in the duct at the center line, and that gave her U infinity. Okay. Then she inserted a
thermocouple in the line that gave T infinity -- thermocouple -- digital readout. We have
that all throughout -- throughout the fluids lab. Then, she attached -- to the outside
of the cylinder, she attached thermocouples. How many? She took -- she attached four -- one
to the front side, one to the backside, and one 90 degrees -- and that gave her T surface
-- digital meter output thermocouple transducer. Okay. And then she varied the velocity, U
infinity, over a range ten times -- ten different velocities. Gave her ten different Reynolds
numbers. Multiply the velocity from the pitot-static tube by the diameter, divide by kinematic
viscosity at what temperature? The film temperature. Average TS. Why do you put three thermocouples
on? Because the temperature around the cylinder won't be the same. They'll be close, but not
the same. Aluminum's a good conductor of heat. They should be close, but not the same. So
you average those three temperatures. Okay. Now you want to find the Nusselt number by
definition -- Nusselt bar means the average is H bar D over K. Get K at the film temperature
-- got it, for air. Diameter. Okay. Four inches -- four divided by 12. Now, the H bar. How
do you get H bar? Well, you go back to Chapter 1. For our convection heat transfer, Newton's
law of cooling, Q equal H bar AS TS minus T infinity. I measure TS -- thermocouple,
average them. I measure T infinity. Got it. The surface, AS pi D L, got it, got it. I
have the watt meter, tells me the input power, got it. Solve for H bar. Get Nusselt bar.
All right. Now I've got it. So now I've got the Reynolds number and the Nusselt number.
Now, of course -- first of all, the person that did this experimentation, and getting
away from the senior project, now. But who came up with something like this? Well, I'll
tell you who did -- the guy that he probably took ME312. Okay. ME312, we say, you know,
if you run an experimental program and you want to plot the results, you can just be
random and plot anything against anything. I think I'll plot U infinity versus the K
value. Bad choice. I -- I -- I think I'll plot the U infinity versus the diameter. I'll
change the diameter. No. Bad choice. We tell you ME312, there's something called dimensional
analysis. They say, if the heat transfer's a function of these different parameters,
you can put them together in dimensional parameters, and that's a major hint of what you, the experimenter,
should do in a laboratory environment to plot your data -- your results. Okay. If you do
that with -- with all these parameters, you take H and -- and D and the properties K and
new and U infinity -- forget this guy. This guy is not serious. Forget him right now.
Then, what pops out? The important dimensional parameters are the Nusselt and Reynolds numbers.
Okay. What does that tell me? If you make a plot from your experimental data, the first
thing you should try and do is plot Nusselt versus Reynolds. Okay. That's why engineers
know that that's called the form of the equation I expect. It's called a power law variation
-- power law. So I would plot -- and she did for her senior project, she plotted on here,
Nusselt bar and the Reynolds number and she had ten different velocities, so she had ten
different Reynolds numbers. I'll just show you a few of them here. I'll just say that's
the range and I'll just make it up based on that 40. Here's 40. Here's 4000. And by the
way, if you're going to try and do a correlation -- a power law, the best thing you want to
do is plot that data on log-log paper because if you plot it on log-log paper and it correlates,
you're going to get a -- you're going to get a straight line -- a straight line. If it's
a power law, you'll get a straight line. Look at that data. You say, I don't think that's
a straight line. I don't think that's a power law variation. I guess my results are all
really miserable. I feel bad my senior project failed. I'm going to go home and cry. Yeah,
right. No. No. Not exactly. You say to yourself, you know what? If I want to be real tricky,
those points there almost form a straight line. Look at them. And those points right
there almost form a straight line. Look at them. So, what do you do? So you know what?
I'll fit that curve piecemeal to a bunch of straight lines and then I'll tell you, the
user, what to choose for C and M for that equation. That's what they do. For instance,
4000 to 40,000. Now we tell our freshman and early classes -- we have them do log-log plots
some times. We say, okay. If you want to find the value of C, go to that graph with the
Reynolds number one. Okay. Put one in there for Reynolds. Get the Prandtl number now.
Reynolds one. Raise it to any power you want. I don't care. Raise one to any power you want,
what do you get? One. One. What's the value of C? The value of the Nusselt number where
the Reynolds is equal to one. Okay. You go over here, between 4000 and 40,000. Go back
to where the -- on your log-log graph where the Reynolds is one. Oh, there's the value
of C -- .193. Got it. And then, what's M? M is the slope. Take a ruler out if you want.
You can take a ruler out. Centimeters. Measure the slope of that line. That slope would come
out to be -- with a ruler, for instance, make it really easy on yourself -- .618. So that's
what these guys do who put together these empirical equations. They get a hint on what
to plot from dimensional analysis, then they make their plot and try to fit it into a power
law if they can -- if they can. You know. So -- and it's nothing new. And now, let me
tell you something, okay. If she would have repeated this with a different diameter -- let's
say she doubled the diameter to eight -- eight inches, here's her data points. She says,
you know what? I'm going to put this thing in a water channel down in the fluids lab,
down there -- the civil engineering water channel. I'll put it in there and I'm going
to measure with water now, not air, water. You know. Guess what? Conclusion. It doesn't
matter what the fluid is, it doesn't matter what the diameter is, it doesn't matter what
the temperature is within bounds, all my data falls on the same line. Oh, you know that
from ME312. That's what we begin [inaudible] ME312. It doesn't matter how you change these
things. If you've got the right parameters from your dimensional analysis, they're going
to all plot on the same line. I'll tell you something else. Here's a Reynolds number,
here's a friction factor, here's 2000. Get the Reynolds number. This is for water. This
is for air. This is for oil. You know what? It doesn't matter what the fluid is. It doesn't
matter what the -- what the diameter of the pipe is. I get all my data points on the same
line. Wow, is that amazing? Yeah. But not really. It -- it comes from dimensional analysis.
What are the important dimensional parameters in the Moody chart? This guy's dimension is
F, this guy's dimension is Reynolds number, this guy's dimension, Nusselt, this guy's
dimension is Reynolds number. The power of dimensional analysis for engineers. It is
phenomenal. Phenomenal. Is there a special Moody chart for water, one for air, one for
oil? No. No. No. No. The same chart, one piece of paper. One piece of paper for everything
in the world. Wow! Power. You got it, man. Power. Same thing here. We end up with a plot.
So -- just so you know. Sometimes people say, I'll use it, but I don't know where it came
from. There's where it came from. Okay. And we had a senior project that did this. Now,
her answers were accurate to these numbers here within 25 percent. Pretty phenomenal
for Cal Poly versus Cal -- Cal Tech or Stanford. Okay. I mean, that's -- that's pretty good,
so. Twenty-five percent, I thought -- good job. You know, you did a good job on that
senior project. But, yeah. She took an idea out of a textbook and based her senior project
on that idea. Okay. Now, let's talk a little bit more -- anything about that right now?
Okay. Let's talk a little bit more about these equations. Now, you say, you know, it looks
kind of strange there. If I use equations one and three --
Properties of T film. But be careful. Be real careful. That guy right there. Properties
at T infinity. Shift gears. Big note on your -- no, not on your equation sheet, the exam.
Because on the exam, only one you're going to use -- that's why I boxed it. The only
one you're going to use on -- in this course for an exam situation is that boxed equation.
Okay. That way you don't have to worry about that. Well, but why are they different? How
come you shift gears in the middle? Well, dealer's choice. Dealer's choice. He says,
if you use my equation, who's mister -- who -- who's that? Mr. Zukauskas, Russian scientist.
Okay. If you use my equation, base your properties at T infinity, except where you see that Prandtl
S. That S means at the surface temperature. Why did he do that? Just because he, you know,
doesn't like other people? No. He found his data correlated better when he did that. His
experimental data correlated better, so he said, okay. That boxed equation is based on
properties of T infinity. Dealer's choice. He took the data, he tells you what properties
to use. Okay. Why are there three? I mean, a lot of textbooks only give you one -- one
equation. Our author -- this textbook is so thorough, they give you three -- three equation
here. This one is -- you don't need a table, which is nice for -- for computer code. You
don't need to go into a table. You -- you can -- one equation covers everything. But
it's -- it's a complex equation. So for our homework and -- and exams, no. No. Before
I forget it. Well, I'll go ahead and continue this first. Why are there three? Okay -- 19
-- this, this -- if you check the references at the back of the chapter, this is -- this
was published in 1977. This was published in 1972. This was published in -- really?
Before World War II. My gosh, all mighty. Geeze. You know, '67, '77, five -- 80 -- 82
years ago. Oh, my gosh. They're still in the textbooks? Yeah. Why? Well, it's so simple.
That's why. This has a little more complexity here to it, you know. Now, that was a simple
one. But when that guy took experimental data -- what did they have? I'll tell you one thing,
they didn't have the -- the handheld calculators. They didn't have the laptops. They had slide
rules -- slide rules. They didn't have the sophisticated instrumentation we have today.
So, these numbers here, oh, they're suspicious. You know, they're -- they're suspicious. I
mean, it's risky. You wouldn't want to use that equation in the real world. No. No. These
two guys here. Say, okay. This textbook, this edition, 2011 maybe, I think it is. I checked
the front cover -- 2011. Eleven plus 23, 33, 34 years ago -- 34 years ago! Both these guys,
roughly. Well, where's the equation 2008? No, it's not in the book. How about an equation
for 2001? No, it's not in the book. 1993? Sorry, it's not there. What does that tell
you? I guess, I'll do a search. You know, I'll do a search and see if I find one. But
if I can't find a newer one, I've got to use this guy right here. He's the latest one.
Probably the best instrumentation. Why isn't there a later one? Well, one good reason may
be -- and this is a maybe, okay. In order to do this research program and do a good
one, you need lots of money -- lots of money. You need a couple technicians -- at least
two or three technicians. You need a couple of people to analyze the data and report back
to you. Oh, a staff of about five, plus yourself. You pay them, you pay for lab space. I got
a wind tunnel here. Wow! You lock it up for three weeks -- a wind tunnel. Wow! Maybe -- maybe
millions of dollars. Probably millions of dollars nowadays. Well, what -- what -- what
-- what -- what -- how come they could do it? Well, I tell you something. If you want
to put a man on the moon, you don't want to use a 1933 data instrumentation. I'll put
man on the moon, I want the latest and the greatest. So I better get new H values. And
by the way, there was something called the Cold War then. Oh, it was -- it was in full
blown, man -- it was full blown. Tons of money dumped into who? Our money. DOD. Yes. Tons
of money dumped in the space program. Who? NASA. Us. Yeah. They dumped tons of money
in to get the latest and the greatest. They didn't want to depend on pre-World War II
instrumentation. No. Slide-rule era. Oh my gosh. No. No. Since then, [inaudible] says,
all right, we need -- we need to get some H data on this for something. What are you
going to use? The guy says, well, I'll tell you, boss, either I can use this 1977 data,
that's the last one I found, or I can do a -- a new research program for you for $1.2
million. The boss says, you know what? Let's just use that one. That's okay. Yeah. Unless
you find money, you don't do the work. That's part of the -- the way we work in this world.
If nobody pays you, you don't do the work. So there better be a darn good reason to spend
a lot of money to do the work. Depends on what you want to do, you know. Obviously,
they said that's good enough. That's good enough. And I'll read you what it says in
the book. The reader is cautioned that all these correlations are only valid within 20
percent accuracy. That's a big -- that's a big one right there. They're only valid in
20 percent accuracy. Wow! Wow! Why is that? Well, let's go -- first of all it's experimental.
So you've got experimental uncertainty. Experimental uncertainty. If you minimize that, then what
else is there? Okay. This guy right here. I'll -- I'll just give you for instance. This
guy right here. You know how hard it is to get that thing to be uniform profile? You
do it in a wind tunnel. Do you think you're accurate on the 10 Freeway has that approaching
the -- the grill? I don't think so. You think it's coming straight towards you? I don't
think so. Not most of the time. So very seldom -- very rarely do you have a uniform profile
that looks like that approaching your radiator because your radiator has circular tubes with
air going over them. A heat exchanger has circular tubes with maybe steam going over
them. So right away, is that steam perfectly like that? Oh my gosh, no. Did you see those
heat exchangers? They go like this. Yeah. Yeah. Over the tubes. So, yeah. That's why
that uncertainty's 20 percent. Just so the -- the reader knows. Be cautious because don't
think you're -- don't carry your answers to eight places accuracy on your -- on your TI
1050 model. It's ridiculous. What -- what accuracy better -- should be on your spreadsheet?
How about three significant figures. Nothing more. Nothing more than that. So yeah, be
aware of that. Okay. So the bottom line in our class, we're using this guy. Even though
he's outdated, we're using him. You get the idea how to find the H values. Problem is
for homework, he uses different equations for different problems, so the answers -- if
you use -- pick and choose, you might be off because he used this and you use that, or
whatever. So I'm going to tell you know what you should use to get the answers for homework
so your answers may match his answers. All right. Chapter 7, problem 7-45, 7-45. I'm
sorry, 7-47 -- 7-47. Use Hilpert and you should get his answer. Problem 7-49, he used Churchill
but I don't want you to use Churchill because it's too complicated for what we're doing.
We're going to use Hilpert. So the answer for H bar or whatever you might get, Q, will
not match his answer -- will not match the author's answer -- 7 -- problem 7-49. Use
Hilpert no matter what. Problem 7-53, he used equation 7-44, so we're okay. That's Hilpert.
Problem 7-53, don't do part C. We didn't discuss fin effectiveness. We discussed fin efficiency,
not fin effectiveness, so don't do part C.
Which problem was that?
7-53. That's the only three problems on this stuff. Now -- all right, now, all this leads
up to how about non-circular? Non-circular tubes and pipes? Well, there's a lot of tubes
in the real world that are non-circular. Look at your automotive radiator. They're not round
tubes carrying water. Look at them some time. So to do that, we use Hilpert again. These
are empirical correlations from experimental programs. We use that with Table 7-3. Okay.
Here's Table 7-3. Geometry. Reynolds number C and M. Okay. So it -- first one, non-circular
is a square tube with a pointed part facing forward. This is dimension D. Square tube
with the flat part facing forward. This is dimension D. There's five different pictures
given. I'll show you three. A hexagon-shaped tube --
with the flat pointing forward. And then they give you -- I'll just give you one here. A
Reynolds number range 5000 to 60,000. Then they give you a value of C -- .158 and .66.
And there's two more pictures given there. So you use the Hilpert equation right here,
and if you want the C and the M, you go over here to get the C and the M. It's very straightforward.
It's the only thing you can do. It's the only choice you've got in the textbook is to use
Hilpert with that. This data here was taken much later than Hilpert equation data so this
is more updated data for C and M here. You say well, what if my Reynolds number is 80,000?
[Inaudible] good luck. It's probably out there somewhere. You got to search it. These are
the most popular Reynolds number ranges. Okay. Question back there?
Why don't they, like, update the table?
This one right here?
Yeah.
That's a good -- that's a good point. That's a good point. I -- I suspect that -- that
this one is updated and what they do with this guy here -- I didn't [inaudible] -- this
-- this accounts for property variations. But when they did this one -- when they -- when
they did this correlation, they did use newer instrumentation. So, yeah. I -- I think this
one supersedes that. If -- look at the form, look at that form. They're identical except
for that property variation. And what if it's air? .7 divided by .72 raised to power -- one-fourth
power. Nothing -- close to one. Okay. So I think that's why they -- it has been updated,
but it's been updated and extended to its usefulness. Yeah. Okay. Now, over here, just
so you know, Reynolds number -- and Nusselt number. D obviously does not stand
for diameter. What's the diameter of a hexagon? No. Don't ask a question. There is none. D
does not stand for diameter. It stands for dimension or distance, take your choice. Dimension
or distance. What is D in these pictures of this tube? It's the distance from the upper
most point to the lower most point -- top to bottom. Top to bottom here, top to bottom
here, top to bottom here. That's how they define the Reynolds number and the Nusselt
number. There's nothing magic about that dimension in the Reynolds number. The Reynolds number
is U -- is a velocity, okay. A velocity times some dimension divided by a property. What
dimension? Well, if it's a flat plate, the dimension is X. If it's a circular tube, the
dimension is D, the diameter. If it's a non-circular tube, it's the distance from the top to the
bottom of what the air sees. Here's the air approaching here. What I see in front of me,
this tube here, is that distance, top to bottom. When they do -- I tell my 312 class, when
they do, let's say, drag force studies on a -- on a Nascar or a top fuel dragster or
a funny car, typically, John -- John Force Racing in Orange County, they -- they plot
the drag force as a function of the Reynolds number. If you're -- they -- they put motorcycles
in wind tunnels -- full-sized motorcycles in wind tunnels with a rider on it and change
the speed in the wind tunnel and get different -- different Reynolds numbers and they get
different drag forces on there. Well, when they do that for, let's say, a funny car in
drag racing, what do you think they're going to find D as? The -- the headlight diameter?
No, I don't think so. The wheel diameter? Why? It's got to be something. I don't know.
Maybe the dimensions of John Force's head? I don't think so. You got to -- you -- I don't
know. What should I choose? What should I choose? Well, what they choose. Here it goes.
The distance that the air sees from the bottom to the top of the car. You face the car frontal
and you measure the distance from the wheels on the ground to the top of the funny car,
and that's how they get the Reynolds number. The motorcycle rider. You measure the distance
D -- the distance from the tires on the ground to the top of the guy's helmet on his head.
Metro link train, you measure the distance D from the railroad tracks to the top of the
metro link engine. A tractor trailer truck with the tractor in front and the trailer
behind, look at it head on, and to get the Reynolds number of that truck at 60 miles
an hour on the freeway, you take the distance D from the tires on the ground of the -- of
the tractor to the top of the trailer, top of the trailer. That's the dimension D you
put in the Reynolds number to get the Reynolds number of the truck going 60 miles an hour
on the 10 freeway. So it -- it -- that's the choice you make. What should I -- what should
I put in for this dimension D in the Reynolds number? Whatever makes sense to you. Whatever
makes good common sense to you. Okay. All right. So just -- so you know how you handle
these guys right here, okay. For instance, I can put -- I can put a -- a square tube
like this -- a square -- it's really a fin. There's a fin. I attach a fin to a heated
surface to take heat out. I attach a fin to a heated surface. This could be electronics
package. I want to take heat out. I put 100 fins like that on this thing and blow air
over it. Okay. What do I do? Okay. Right here. I use -- I use that equation in the box. I
use this for D, which is this dimension right here if it's square. I put it in here. I get
H bar. I put the H bar in here. I get Q. That's the heat loss by what? By one fin. Multiply
it by 100. I get the heat loss by 100 fins plus the base area. The heat loss of the base
area here, too. So that's how they use stuff like that. Okay. It -- it can be a fin or
it can be a tube carrying water with steam blowing over it, whatever. These guys right
here are condenser tubes. And they could possibly, I'm just giving example -- condenser tubes.
Steam hits them. Cold water inside. The cold water condenses the steam, the steam drops
off, goes to the hot well. That's how we engineers analyze these things like this. Okay. Good
stopping point. We've finished Chapter 7. Okay. I'll do an example on Friday for you.
But we're through with that right now. And let me go pass out the -- the second exams
for you.
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