Discussion Forum

Posted: 1 decade ago 10.03.2012, 09:01 GMT-5

Hi Interesting question, as I suppose you do not want to simuate explicitely convective fluid flow inside your glass square, only air conduction + the lumped convection term. By the way you ave the same issue on your boundary "2", no ? After some thought, I would have drawn another concentric circle, in the internal air domain, about midway between boundary 1 & 2 and use the temperature on that internal air boundary as input for your T_ext. As then you loop your model such that you will make it non linear, but it should still work. You can set a cylindrical coordinate at your circle centers and project with an operator the temperature along a radius from this mid boundary onto your boundary 1 (& 2 for that sake). Else "T" which is a field T(x,y,t) cannot be adressed correcly -- Good luck Ivar

Posted: 1 decade ago 12.03.2012, 05:09 GMT-4

Hi and thank you for you analysis, indeed, i'm interested only on air conduction and (solid to fluid or fluid to solid) convection. Actualy, i need time to think regarding boundary 2. i should compare temperature on cooper and temperature on boundary 1 with laboratory mesurement. so boundary 2 has very small influence to cooper and boundary 1, i think. imagine if there is no glas cube. T_ext for Convective cooling at boundary 1 is the coolest temperature of air around it, right? with glas cube, the coolest temperatur ist temperature that close to boundary 2. for convective cooling at boundary 2, t_ext should be the hotest temp near to boundary 1. heat transfer coefficient = Nuselt (Rayleig, Prandt, grashof). Air properties are taken at (Temperature_solid + temperature_fluid)/2. If T_ext taken from midway between boundary 1 & 2, it is warmer than i think should. less heat are transfered. please more discussion, i'm happy to hear any idea about this. best regards akmal

Posted: 1 decade ago 12.03.2012, 06:11 GMT-4

Hi indeed depending on your overall conditions mid way is justa first guess could also be boundary 2, but boundary 2 exchanges also with boundary 1. The thing is that a convective boundary assumes a heat sinc at Tconvective, and you have true convective echange solid-fluid-solid, this means that to simulate that correctly you ideally must take into account the heat flow So this tells me that convective assumptions on these internal boundaries are not correct as some heat flux will "escape" the model, or have I misunderstood something ? -- Good luck Ivar

Posted: 1 decade ago 12.03.2012, 13:30 GMT-4

Interesting problem. Is the temperature of the copper known (and therefore imposed as a boundary condition), or is the copper a heat source (for example it carries a certain current), and generates heat that must be dissipated. If the latter, then boundary 2, boundary 1 and the air gap between the PVC and the glass will all have a bigger influence on the temperatures everywhere. I prefer setting up problems like this as a conjugate heat transfer problem (heat transfer with CFD). The reason is that it is not easy to accurately represent most enclosed fluid regions using convection equations. Both the heat transfer coefficient “h” and the external temperature “T_Ext” are problem dependent. The heat transfer coefficient is influenced by how the air flows around the object, and there are different analytical/empirical expressions for different cases. Similarly, “T_Ext” is based on the expression used for the heat transfer coefficient. In the case of external natural convection it is the temperature of the ambient. For enclosures it is usually the temperature difference between two walls of the enclosure (each at a constant temperature). Two more factors for you to consider when modeling convection in enclosed air regions. The first, which Ivar mentioned, is to watch out for heat that incorrectly escapes (or enters) the model by convecting out of boundary 1 and not reaching boundary 2. The other factor is radiation. For well insulated regions with low natural convection (h

Posted: 1 decade ago 14.03.2012, 06:12 GMT-4

[QUOTE] Hi indeed depending on your overall conditions mid way is justa first guess could also be boundary 2, but boundary 2 exchanges also with boundary 1. The thing is that a convective boundary assumes a heat sinc at Tconvective, and you have true convective echange solid-fluid-solid, this means that to simulate that correctly you ideally must take into account the heat flow So this tells me that convective assumptions on these internal boundaries are not correct as some heat flux will "escape" the model, or have I misunderstood something ? -- Good luck Ivar [/QUOTE] Hi Ivar, Yes, your are correct. heat flow should be observed strickly especialy for "boundary 2". @ Nagi, thanks for your ideas. indeed, it is a wire model, high temperature are produced by resistive loss. at the both ends there are difference conditions (e-machine and inverter module). for "h" is formula for horizontal cylinder. "For enclosures it is usually the temperature difference between two walls of the enclosure (each at a constant temperature)." for stationary case, yes. But for time dependent where both walls has "mutual interaction", not really. i'm still on my opinion: for convective cooling on boundary 1, T_ext is temperature that close to boundary 2. for convective cooling on boundary 2, T_ext is temperature that close to boundary 1. what do you think Nagi? of course by using CFD is the modell more realistic. i'll try on 2D-modell. regarding radiation, the emissivity of both walls is unknown, i'll using an assumption (emissivity for boundary 1 = 0.6, boundary 2 =0.9). h at boundary 1 is minimum 10,5 to max 13 W/m²K. at boundary 3 h is 9,5 to 10 W/m²K. best regards akmal

Posted: 1 decade ago 14.03.2012, 11:32 GMT-4

Hi Akmal, The assumptions you stated for the ambient seem reasonable to me, though I believe that the walls have mutual interaction in both the transient and steady-state solutions. Make sure you monitor the heat leaving/entering both convection boundaries to make sure there is no unintentional heat gain/loss. Regarding suitable formulae for “h”, there are two possible candidates one based on free external convection from a horizontal cylinder and the other based on convection between two concentric horizontal cylinders. Nagi Elabbasi Veryst Engineering