7 replies to this topic

[Jaldhipatel]

Hello..is there any relation between fluid pressure and heat transfer..?

[MrShorty]

Since your question is very broad and not very specific, I will grab the low hanging fruit. Yes, there are some obvious, broad relationships between fluid pressure and heat transfer.

 

For example, One insulating strategy (in dewars and similar) is to use a high vacuum in a double walled vessel. High vacuum does not transfer heat very well. But, it also seems important to maintain the high vacuum, because it doesn't take very much air/pressure leaked into the insulating space for heat transfer to increase. So, in a gas at high vacuum, it seems that heat transfer can be a strong function of pressure.

 

From there, you probably need to be more specific in your query. I expect there are ways in addition to considering the high vacuum case where pressure can strongly impact heat transfer rates, and other scenarios where pressure will have a much smaller impact on heat transfer rates. Share with us the heat transfer equation(s) you are using or the situations you are considering, and we can provide more useful thoughts.

[breizh]

Hi,

Can you elaborate ? what is the context ? Is it related to a specific problem ?

 

To help you we need you to help us .

 

note : same for your second post , your description is too vague .

 

Good luck.

 

Breizh 

[Jaldhipatel]

In case of chilling or cooling what effect of water pressure on heat transfer/ heat transfer coefficient...is nusselt number may help..??

[MrShorty]

Wikipedia (https://en.wikipedia.../Nusselt_number ) says that Nusselt number is the ratio of the convective to conductive heat transfer in a fluid NuL=h*L/k where h is convective heat transfer coefficient and k is the thermal conductivity and L is the characteristic length.

 

You can use your favorite fluid property calculator to calculate how thermal conductivity changes with pressure (this one works well for water: https://webbook.nist...hemistry/fluid/).

 

According to this essay at engineering toolbox https://www.engineer...sfer-d_430.html

 

Convective heat transfer coefficients - h_c - depends on type of media, if its gas or liquid, and flow properties such as velocity, viscosity and other flow and temperature dependent properties.

Pressure is not directly mentioned as an important variable, but I could see pressure (as the driving force for fluid flow, if that is applicable as well as a determining factor in whether the water is gas or liquid) being indirectly implicated in the heat transfer coefficient.

 

With that in mind, I expect if you did the calculations assuming unchanging flow rates/properties, you would find that:

1) Nusselt number is a weak function of pressure below the saturation pressure of water.

2) There is a large jump in the Nusselt number at the phase boundary.

3) Nusselt number is a weak function of pressure above the saturation pressure.

 

At this point, I would recommend that you fill in the details from your actual heat exchanger operation. If you like Nusselt number as a way of quantifying the effect of pressure, then determine the Nusselt number at different pressures. Then you should be able to judge how strong or weak the effect of pressure is on Nusselt number and your operation.

Edited by MrShorty, 16 June 2021 - 10:29 AM.

[Jaldhipatel]

Thank you mr shorty. I learned new things through your reply. I think this from heat exchanger designing side also...in heat exchanger design we use generally seader tate or ditus bolter eq. To calculate heat transfer coefficient. And in that co-relations we have renolds no and prandlt no available...so it's useful to get idea about pressure of fluid and heat transfer relation..pls share your ideas..

[MrShorty]

It might be better to get the opinions of someone more experienced with heat exchanger design, but I don't see anything in those equations that changes my conclusions. Seader-Tate and similar equations are functions of Reynold's number and Prandlt number and viscosity -- none of which seem to directly reference fluid pressure. Even using these equations, the effect of pressure seems to mostly be indirect. Pressure can have an effect on various fluid properties (like density, viscosity, or thermal conductivity) and it can change the speed and nature of the fluid flow.

[breizh]

Hi,

Let you Google : Effect of pressure on heat transfer 

you will find some information .

Good luck

Breizh