heat flux and heat transfer coefficient

Prabin Pradhananga_22497

Hi guys,

I was going through heat transfer tutorial and have a slight confusion with the following:

What is the significance of using both heat flux and convective heat coefficient here? And why heat flux is set zero? Do I need to define both of them? Or, can I use either heat flux or convective heat transfer coefficient?

The link is:

https://2022.help.altair.com/2022.1/hwcfdsolvers/acusolve/topics/tutorials/acu/acu_3110_intro_cfd_r.htm#acu_3110_intro_cfd_r

Thanks!

acupro

Prabin Pradhananga_20428 said:

Thanks!

The reference temperature is given by the user. And, the surface temperature of the body exposed to surrounding is calculated by the algorithm, right? Is the q (W/m2) in the formula below different than the one in the dialog box above?

 

The 'Heat Flux' entry is for a user-applied heat flux on that surface - in W/m2.  For example if you have a heat tape applied to the surface and you know what the output is.  Or if you know the output of some attached device, not being modeled, being applied to the surface.  The default for 'Heat Flux' is zero - meaning no additional applied heat flux.  If the surface is the boundary of the domain, this indicates an insulated surface.  If the surface is the boundary between two volumes, it would yield the natural heat flux across the boundary due to material properties and temperature difference.

The convective heat flux coefficient (or convective heat coefficient) along with the reference temperature (or convective heat reference temperature) would allow the convective heat flux behavior per the equation you indicate above - h being the coefficient and T_inf being the reference temperature.  This would allow for any assumed convection heat flux from that surface, using those assumed parameters - coefficient and reference temperature.

Jagan Adithya Elango

Heat flux in the heat flow rate intensity. Heat transfer coefficient is the effectiveness of heat transfer which depends on heat flux and temperature difference.

 

yes you can use both on a surface where heat is being generated and dissipated. (i.e. brake disc where heat is generated because of friction (heat flux) and dissipated to the surrounding air (convective heat transfer coefficient)

Prabin Pradhananga_22497

Jagan_21383 said:

Heat flux in the heat flow rate intensity. Heat transfer coefficient is the effectiveness of heat transfer which depends on heat flux and temperature difference.

 

yes you can use both on a surface where heat is being generated and dissipated. (i.e. brake disc where heat is generated because of friction (heat flux) and dissipated to the surrounding air (convective heat transfer coefficient)

Thanks!

The reference temperature is given by the user. And, the surface temperature of the body exposed to surrounding is calculated by the algorithm, right? Is the q (W/m2) in the formula below different than the one in the dialog box above?

 

acupro

Prabin Pradhananga_20428 said:

Thanks!

The reference temperature is given by the user. And, the surface temperature of the body exposed to surrounding is calculated by the algorithm, right? Is the q (W/m2) in the formula below different than the one in the dialog box above?

 

The 'Heat Flux' entry is for a user-applied heat flux on that surface - in W/m2.  For example if you have a heat tape applied to the surface and you know what the output is.  Or if you know the output of some attached device, not being modeled, being applied to the surface.  The default for 'Heat Flux' is zero - meaning no additional applied heat flux.  If the surface is the boundary of the domain, this indicates an insulated surface.  If the surface is the boundary between two volumes, it would yield the natural heat flux across the boundary due to material properties and temperature difference.

The convective heat flux coefficient (or convective heat coefficient) along with the reference temperature (or convective heat reference temperature) would allow the convective heat flux behavior per the equation you indicate above - h being the coefficient and T_inf being the reference temperature.  This would allow for any assumed convection heat flux from that surface, using those assumed parameters - coefficient and reference temperature.

Prabin Pradhananga_22497

Hi all,

I actually want to know more about how acusolve handles no slip boundary conditions with heat transfer. As far as I know, we can have following thermal boundary conditions for no slip walls: 

1. fixed heat flux
2. fixed temperature
3. convective heat transfer
4. external radiation heat transfer
5. combined external radiation and convection heat transfer

But, what if I specify both heat flux and convective heat transfer coefficient? Does this over define the system? Some might say heat_flux here is due to volumetric heat generation (source). But I am still confused with this interface in hyperworks cfd. Please correct me if i am wrong.

Thanks.

acupro

Prabin Pradhananga_20428 said:

Hi all,

I actually want to know more about how acusolve handles no slip boundary conditions with heat transfer. As far as I know, we can have following thermal boundary conditions for no slip walls: 

1. fixed heat flux
2. fixed temperature
3. convective heat transfer
4. external radiation heat transfer
5. combined external radiation and convection heat transfer

But, what if I specify both heat flux and convective heat transfer coefficient? Does this over define the system? Some might say heat_flux here is due to volumetric heat generation (source). But I am still confused with this interface in hyperworks cfd. Please correct me if i am wrong.

Thanks.

Yes - you can specify both of those.  These are boundary conditions to account for effects not included in the meshed domain of the simulation itself.

Example:  a heat tape with known input is applied to the meshed model's surface, and the meshed model is also sitting in a room (not meshed) with a known average room temperature and 'natural' air currents.

The applied Heat Flux on the BC accounts for the known input to the heat tape.  The convective heat flux coefficient and reference temperature account for the air 'currents' and room temperature of the air outside that is not meshed.

Or maybe the tape applied to the surface cools the surface instead of heating it - then the Heat Flux in that BC panel would have a negative value.

Similarly, if radiation is active, you could also add a radiation BC as you have stated, accounting for heat transfer between the surface and some temperature 'far away'.

Prabin Pradhananga_22497

Hi all,

Can you please explain about the sign of the heat flux correctly? 

Is +ve heat flux means => normal to the heat flux is directed towards the domain?

Is -ve heat flus => normal to the heat flux is directed outwards of the domain?

Or, sign of temperature gradient has some role to play?

Thanks

 

Sebastian Yang

Prabin Pradhananga_20428 said:

Hi all,

Can you please explain about the sign of the heat flux correctly? 

Is +ve heat flux means => normal to the heat flux is directed towards the domain?

Is -ve heat flus => normal to the heat flux is directed outwards of the domain?

Or, sign of temperature gradient has some role to play?

Thanks

 

As far as I know, from my experience. If the heat flux is positive, the heat enters the domain, if it´s negative, it goes out. It is easy to set up a model and try it yourself and you will find it out.

Prabin Pradhananga_22497

Sebastian Yang said:

As far as I know, from my experience. If the heat flux is positive, the heat enters the domain, if it´s negative, it goes out. It is easy to set up a model and try it yourself and you will find it out.

Thanks!

But I noticed that when I specify negative heat flux for top surface in the input file, the surface heat flux plot in hyperview shows it positive. I have attached the following images--it can be seen that the red region is +2000, but in fact, I have specified -2000 in the input file. Why is this happening?

Sebastian Yang

Prabin Pradhananga_20428 said:

Thanks!

But I noticed that when I specify negative heat flux for top surface in the input file, the surface heat flux plot in hyperview shows it positive. I have attached the following images--it can be seen that the red region is +2000, but in fact, I have specified -2000 in the input file. Why is this happening?

Hi, I am not sure about the sign of heat flux in the results. I had previously made tests but I only focused on the temperature distribution, I mean I checked if the temperature around the surface where there is heat flux is higher than the adjacent cells or is lower, then I determine if I had put the sign of the heat flux correctly. 

Can you check that?

Also after checking my problem again I find out that the surface heat flux of the surfaces where I defined a positive heat flux, in Hyperview it´s negative.

Prabin Pradhananga_22497

Sebastian Yang said:

Hi, I am not sure about the sign of heat flux in the results. I had previously made tests but I only focused on the temperature distribution, I mean I checked if the temperature around the surface where there is heat flux is higher than the adjacent cells or is lower, then I determine if I had put the sign of the heat flux correctly. 

Can you check that?

Also after checking my problem again I find out that the surface heat flux of the surfaces where I defined a positive heat flux, in Hyperview it´s negative.

Hi,

Yeah, I also found that when i defined a positive heat flux, it shows negative surface heat flux in hyperview.