Thermodynamics - Heat Transfer vs. Velocity

Sloppy Joe:

Velocity in relationship to heat transfer.

Yes velocity would increase heat transfer rate, with the transition between laminar flow to turbulent flow, which is defined by a dimensionless number called the Reynolds number.

Now this Reynolds number can be derived by a number of different ways, and the point of where a most (i can't think of a good word to use at this moment, so I'll use this one) efficient point of heat transfer, And this can vary with the physical properties of the product you are using.

As well as the condition of the inside of the pipe you are using, and the size, You may even need to put in some kind of insert-able core to keep the product near the transfer surface.

It can get very intense calculations, to derive a most efficient rate of heat transfer.,

But can you see the little bit I gave their are quit a lot more other factors involved.

phoenix911

First obtain Reynold's number from;

Re=Vl/v where, V is the velocity in m/s; l is the length in the direction of flow in meters; and, v is the kinematic viscosity of the air in m²/sec at the given temp.

From this derive (h) heat transfer co-efficient where,

h=0.332x(k/l)x Re ^0.5xPr^0.332 where;

k=Thermal conductivity of air at given temp in W/mK;

l= Length in direction of flow in meters;

Pr= Prandtl number of air at given temp

You get h in W/m²K and average heat transfer co-efficient will be =2xh

I hope this gives you enough inputs.

Sorry, the average heat transfer co-efficient will be :

The internal area of pipe x h

please correct

Thanks for all that information and formula and I did make the correction as you suggested.

With all that said (and I haven't crunched the numbers yet) do you feel that there is ever a point where the velocity could be too fast to acheive optimum or the most efficient heat transfer through the pipe wall?

You would not like to go supersonic?

RHABE

Sloppy Joe,

You know air is a bad conductor and therefore the heat transfer will take place only at the place where it comes in contact with the pipe walls, then through the metal body of the pipe and to whatever medium that is existing outside the pipe including the temp over there. Increasing the velocity will increase friction on the wall and therefore there will be less heat rejection to the wall. I think you are trying to ascertain the 'resident time' that the air will be in contact with the pipe surface so that there is fairly good heat transfer taking place.

Since only the molecules of air near to the metal body will undergo heat rejection, it may be worthwhile to consider some kind of fins inside the pipe so that contact area is increased. Considering that the pipes are huge in dia (8"), you have a very inefficient system for heat transfer. I will be more comfortable by first optimizing the contact area then worry about the optimal resident time.

Still, I'll keep your concern in mind and if I come across something, I'll let you know.

My thoughts exactly.

Take the coolant system of an engine..........the coolant pump impeller is usually a semi enclosed impeller with either straight vanes or short curved vanes. This makes the pump inefficient to slow the flow rate of the coolant.

We then have a thermostat..........one of its main functions is to control or reduce the flow rate of the coolant.

If the flow rate of the coolant is too fast, the coolant does not remain in contact with the cooling surfaces for a sufficient length of time to absorb heat.

If you remove a thermostat from the coolant system, the temperature gauge will indicate that the engine is running cold..............however the reverse is true, the engine is running hot.........coolant flow is too fast........not absorbing any heat from the cooling surfaces, as you have removed one of the elements controlling flow rate, to wit, the thermostat.

it may be worthwhile to consider some kind of fins inside the pipe so that contact area is increased.

or something to create/induce a turbulent flow

You are right Phoenix911

Heat transferred = mass flow rate * surface area for transfer * heat capacity of air* (Tair-Tsurrounding)

u can increase flow rate (velocity), surface area (with fins as suggested by capri), or increase delta T.

I am assuming the temp of hot air is fixed and that of surrounding. So, that will be bottleneck for ur heat transfer assuming surface area can be increased as needed. So increase ur velocity till ur delta T gets lower. When delta T stops getting any smaller, u have reached the limit.

There are several factors such as pressure drop across the pipe (aka pumping power) that will be the optimization parameter.