Wind Speed Cooling Factor?

Sorry, I forgot to say the magic word: Yes Please

More on objects. I'll try the link, thanks Pete. Thought you might come up with something.

I did that fly over before and was getting nowhere. I get sidetracked too easy. Not as simple it seems, Ky.

I just checked your links Peter, and was very impressed. Wish I had gone to the trouble before I posted MY comments, because clearly my comments merely repeated your links.

Oh, I forgot to mention but you probably are already realize this from how you phrased your question. Since a person's body gets also cooled by the evaporation of sweat, there will be a considerably higher amount of heat transfered by the evaporative cooling that occurs with moving air over a person than by just convection cooling. This is why people feel cooler being down wind of a fan that is moving the same temperature air.

For you and others I found this.

It looks like it could be calculated but I am not skilled in that area. A diagram like the above would help. Simply temperature drop and speed of air (fluid) over a surface. In grades from horizontal to vertical. I know that in open air ambient temperature will be the outcome of convection cooling, if in the shade.

What kind of wind speed is required to have the surface at ambient, shade temperature?

I'll do a bit of further searching. Just thought this would explain what I am after.

Coming to think of it, I might take my car and do a test on the change of temperature in what time. 60kmh is the limit on this Island but it should show me something to correlate.

Many thanks, Ky.

Be very, very careful when trying to analyze a phenomenon from a log-log plot. All too often I have seen log-log plots used to over-simplify an analysis and as a justification to discard the "higher order" terms. Redfred has done a good introduction to the complexity of the problem, and I have little to add, other than the "evaporative" effect involves a material phase change- it is going to be different for "sweat" than it is for distilled water, and your plate of spaghetti is going to have a different rate of phase change than your sweaty body, because you are dealing with a totally different set of volatile compounds. That is why the "Higher Intelligence" provided us with CFD analysis tools!

That is why the "Higher Intelligence" provided us with CFD analysis tools!

Hi Charlie

The provision of said tools has past me by. All I could see and remember was a price tag and then I fainted. I mentioned in an earlier post that this inquiry is more about understanding the concept and methodology of taking and correlating such measurements and then comparing it to the real world, my application.

The lab which is going to do this testing has all the tools one could think of. I just want to know what and how they are using them to justify their bill. If I can come up with a ball park figure, log-log plots, which would warrant, demand such tests, I would feel better.

That even the lack of a long enough stretch of road would be part of the equation never occurred to me. All I have are three Infrared laser pointer thermometers and a car. I could just use a fan to replicate the wind speed but then I would require an anemometer which would have to be very sensitive and even detect minute air movement.

To purchase such equipment, for a one off use, is even for me a bit over the top. I appreciate what you and others are stating but the tyranny of Island living takes its toll. I'll get by and my knowledge of the matter has already increased in such a way that any tests in the future will be fully understood by me.

BTW, again, this has nothing to do with the envisaged application. The results will not interfere with the over all system I am working on but make it a bit "sweeter" for any clients to understand. In other words I know the results but can't prove how I got them.

This thread has given me great information about the subject and I can only congratulate CR4 members for allowing information to flow in such a friendly and constructive way. I see this happen to many others (finding solutions to problems) and wonder why it is that hardly a thank you goes out to the participants in these threads. Has politeness gone out of fashion? GA's are sometimes just not good enough.

No if's and but's, I like it here. Thank you, Ky.

Ky,

We have about a twelve hour difference so banter and feed back can easily take days but I have a few suggestions for your testing. (Why am I up so late?)

The infrared laser pointer thermometers only measure surface temperature, they cannot measure core temperature. Nothing wrong with surface readings, as long as you know that they are surface readings. So depending on what you are cooling with the air flow, you can have warmer, cooler or identical core temperatures compared to any of the surfaces measured.

Next a calibrated anemometer can be a considerably expensive instrument and when needed invaluable to an experiment. In your case though I do not think that you need such a precise device. If you can fashion a child's pinwheel with a reflective opto-coupler to measure the blades of the pin wheel spinning you will get a relative wind speed measurement by comparing the frequency output of the opto-coupler.

Mr. redfred, Although I'm not the OP, I've observed and appreciate the intelligent and logical responses that comes from your end. They kind of trigger some olden days of school memories! Thank you, vsar - NYC

Thank you

Thermodynamics is one of those things that can seem so simple, but quickly turn into a chaotic jumble.

Yep, I found that while I was searching. That is why I had asked for some graphs which would give me a better understanding of the convective transfer coefficients. At least I know now what to call it.

I'll see what I can find in your links. Great help. Thanks, Ky.

red fred made a very good presentation of the heat transfer theory.

Now e few practical data: natural convection in still air about 8...10w/m²K and depends on the surface position: lower for horizontal and higher for vertical. The presence of dirt will strongly reduce the thermal flow.

Radiation is small because a low temperature since radiation is proportional to the difference of the 4th power of temperatures.

If the car moves the air speed on its surface is not the same overall. The heat transfer from inside to outside will also depend on the conduction of the different "walls" : a window pane will not transfer as much a steel sheet. For the steel sheet the limiting factor will be convection and for the glass or plastic part the limit can be conduction. so that convection will not be the only factor if you want to look in detail. If you consider the car as a whole, speed will have an effect and you can consider a convection of about 100...200 W/m²K but only as guide line. Of course this value changes with speed increase since the boundary layer thickness decreases.

The picture you put in is a measure of pressure losses in ducts as function of Reynolds number. If you look where links were given you find a lot of informations about all details.

Thanks nick name and woodpower

You are right, redfred did a good job putting me on the right track and with your follow up the ball is now in my court. I'll keep it for a while and get back to you once I have tested my scenario. Although I will be testing with the car the application is for something different.

No luck at the moment though. We had the wettest September since records began so not much of the sun to be seen. We have a mile long stretch of straight road here and I'll just go back and forth at different speeds and start with the same temperature at the some position, recording the difference. I'll make a graph and Bob's part of the extended family.

I'm doing this so that the professionals, who will be dealing with this at some stage, can't say anything I can't understand or have experienced, dealt with hands on. Like I said, the car is only a model, it could have been a spoon with hot soup.

Thanks to you I get the drift (pun, what pun?)

I'll be down the Island drag stretch and give it a go. What would I give for a longer stretch of road. I might brake the "Cool Down Land Speed World Record For The Shortest Distance". We can't just have it all, can we.

Great help from all, Ky.

Well, I had my fun. After going down that stretch of road nothing changed. Only after traveling 7km did the temperature change from 61 C to 40.5 C. Like I said, I could not drive faster without annoying Brendon, the local copper. I'm surprised how much the difference is after such a short ride.

I'll fine tune it over the next days. Always at the same time. It beats working with the complexity of the formula approach and will give me the results I am after, somewhat.

A bit of history. The wind chill phenomenon was developed by Paul Siple when he was a Boy Scout in Antarctica. (I actually met him once--some relatives attended my Church and he was visiting.)

It wasn't important to Paul, but the same thing will apply to any temperature surface--the faster the air movement, the faster heat will be carried away. It actually also applies to warming cold objects; the moving air will carry away the boundary layer and provide faster warming.

Nice one. I found this after googling Paul Siple

I was after something just for warmer temperatures. See what comes up.

"Heat Transmission" by McAdams refers to a paper by Pannel J.R.:

Pannel, J.R., Brit. Aeronaut. Research Comm., Mem. 243 (June, 1916)

This describes flow of air inside a heated brass tube.

This paper:

Hilpert, R., Forsch. Gebiete Ingenieurw., 4, 215- 224 (1933) gives graphs for air flow outside tubes.

Bioramani

I don't think anyone suggested the temperature of the ambient air as a significant variable in the rate of convective heat transfer. It is certainly true that a car moving at highway speeds will lose energy faster to the air it moves though if the air is at 20 degrees F than it will if the air is at 90 degrees F.

In doing the empirical tests, it might be a good idea to record the outside air temperature as well as the speed and air temperature inside the car. This is a more significant factor than the air films or natural convection - all true, but having a relatively minor effect compared to the ambient air temperature.

And, if the issue is the comfort of those in the car, relative humidity and ventilation in the cabin are major contributors along with interior air temperature.

ga.. would love to hear more of the story.

Very interesting application. Similar things were done in desert areas here in Australia. There have been a few posts on this in the past. To condense the humidity from thin air.

Using the colder waters to utilize the higher difference in temperature in a marine situation makes sense. In some way I am taking advantage of the same principle, in some way.

I name parts of systems by names only understood by people in the know. The Helena valve for example. Or the Dali tube. The Christury chamber comes to mind. Not even Chris knows were that is. Never had a teddy but it must be the same feeling I suppose. Mine might not be as cuddly too.

The English Rose effect. Maybe one day, Ky.

You are correct in your statements but you do not understand the dynamics of the stated problem. Let me set up a practical thought experiment.

Take five identical lab thermometers that can be suspended from a string in the room so that the bulbs don't touch anything but air. Suspend one from the string and place the other four in a beaker of boiling water (actively boiling by a burner not just boiled). Say that the air temperature displayed by the room thermometer is 25.0°C and the four in the boiling water all show 100.0°C. Take two thermometers from the boiling water and dry the bulbs with a towel and suspend it from the string. Take two thermometers and suspend it from the string without drying the bulbs. Set up a fan that blows past one dry bulb and one initially wet bulb. Eventually all five thermometers will display the same value, the temperature of the air. But which thermometer will reach 25.5°C first, second, third and fourth. When did they reach these temperatures. Also why will it reach that temperature first and what do you need to predict how quickly it will reach 25.5°C. This is the essence of the stated problem.

Let's not forget that this is a differential problem. So theoretically the four thermometers will never actually reach the cooler temperature of the room. But our measuring scale resolution may not be able to show the difference.

excellent demo. makes sense.

so you are saying that wind chill is really a 'rate of change' problem. So why does wiki not say that, instead of posting the table of wind chill figures when it is basically a 'feels like' temp?

chris

This is different from wind chill since the human body will try to maintain a constant temperature and burn fuel and modify blood flow to accommodate for changes in energy drawn. So one could more accurately say that this experiment matches the wind chill rates of energy transfer only when the cooling thermometers are at body temperature. But as the thermometers cool, they will have less energy to transfer regardless of the mechanism.

Also the metabolic ability of a person to burn calories will change with age and health. So remember the exposure time to frostbite under identical conditions can and will change for each person. The chart shown is just a guide, not am accurate prediction.

Yep, it looks like you have been sitting next to me in the car. Very relevant information.

Now this is an easy one.

I knew somehow that this opening sentence would get me into trouble but I'm on my way out and and happy that my optimism got me to this point. Nothing a bit of study can't get under control.

Thanks redfred, Ky.

You damn me with faint praise, and proceed to obfuscate the solution to the problem.

" but you do not understand the dynamics of the stated problem". What part of the problem did I not understand? The part where I pointed out there is no moisture on the substrate and therefore no wind chill can take place, or the part where I pointed out that air flow over cooling fins improves their performance?

I note that you still didn't answer the OP's question, (Are there tables to determine how fast a dry body will cool down according to wind speed?) but at least I pointed the way. You have proved to my satisfaction that you clearly know the difference between wind chill and differential cooling. Many people do not. The results may be similar, the mechanism is vastly different.

Good thought experiment though. Entertaining. Always a pleasure reading your comments.

Oh,i smell out something here.

yes... surface area.

with the wet bulb temperature device, both the surface area of that bulb, and the wetting of that mesh are increased, giving rise to a near perfect and near maximum 'wet bulb' temp.

As mentioned previously, ambient temp is a factor, but it is covered by T1-T2, because ambient is only half the equation. It is the difference that affects thermal transfer rates in conductive and convective analyses.

Chris

yes... surface area.

You should see the grin on my face.

You know? These tin foil hats? Could they not have some holes in them so that there is a better convection of the hot air? Or is it that the imprisoned hot air that does the trick?

It's turning into a smirk by now

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All the best for the baby, Ky.

Good thinking Yusef

I use three different digital thermometers which are not subjected to the air flow. My biggest problem is the lack of distance I can travel at a reliable speed. Unless I find a theoretical solution I will have to go to Australia (The larger Island west of here) and do tests there and correlate afterwards.

Thanks for contributing, all very helpful, Ky.

Dear Sir, It has to do with coefficient of convection, which increases with velocity of air.As the speed increases the coefficient of convection. The mass of the air which passes over the surface is responsible for mass heat transfer.All text books will give the formula e

kiran.

In hot gas ducts (temp of 900 to 1000 deg C) we found that the flap of a butterfly valve which was always operated partially open would force excessive gas flow over one area in the duct causing a hot spot that was almost impossible to overcome.

Same thing in a flue gas duct (temp of 1250 deg C) - one transition piece always was more of a problem - just plain so many more heat units on the refractory surface we eventually decided.

This is simple:

Energy balance: 1st law of thermo

Heat in(friction) = Heat out(Convection+Conduction)

Heat as a consequent of friction of air and the contact surface (Heat in) = (Heat out) heat being convected to the air and some portion left conducted in the surface or material.

If the speed is not enough to generate heat by friction, the surface will be in equilibrium with that of the air, however at a very high speed just like reentry of space craft on earth, heat due to friction will be at significant amount.

You want to know at what speed does this phenomena occur, well you can play with the formula,

definitely, factors will be dependent on friction coefficient between mediums in contact, contact materials , contact surface area and speed. And to correct you, its not by speed alone it varies.

And, you may not wonder, why the space shuttle was designed to reentry earth at its tip, and why the tip is made up of special materials dissimilar with its body.

This is a good presentation to analyze your problem, noting PV=mRT, where pressure is directly proportional to temperature. Cooling is somewhat dependent, not only by convection but also by geometrical aspect