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Heating water - fluid sub-domain temperature dominates results?

Question asked by Chris Phelan on Jun 27, 2018
Latest reply on Jun 28, 2018 by Chris Phelan
  1. Greetings. I am trying to simulate the process of heating water on a stove, with the end goal being the heat flux through the sheet metal pan below the burner/flame. I already have empirical test results which capture pan temperature vs. time which I have been using to ground my simulations in reality. So far, they are not grounded.

 

To "model" the flame, I have a solid that roughly emulates the flame shape. I have no material defined, so it defaults to air. I've defined the flame as a heat source (given that I know the BTU/hr output of the burner - at least per the spec sheet) and also as a fixed temperature source (1300degC). More details below.

 

To model the pan, I've copied the pan geometry and distance from the flame, and set the materials/radiation surfaces (stainless steel, glazed porcelain).

 

Otherwise, in general, all simulations are external with a defined domain enclosing the model. Heat conduction and radiation are on. The environment is set to 25degC, gravity is on, humidity is on (80%), and the default fluid type is air. I do not have any boundary conditions. Initial ambient is also set to 25degC and 101.3kPa. Goals are set to pan surface temperature (solid) and pan surface flux.

 

All simulations are static except the last one

 

First simulation - no pot or water. Flame modeled as a heat generator with a 17kbtu (4982W) output. Flame surfaces defined as blackbody walls. Pan temperatures exceed empirical results by about 2x. This raises the question - how is heat lost from a heat generating source? I'm assuming it can be shed to the environment - not just the solids defined.

 

Second simulation - same as first, but with the radiation properties dropped from the flame surfaces. Same results as the first simulation - radiation property definition does not seem to impact the results – maybe not too surprising since it is defined as (I’m assuming) an ideal heat source

 

Third simulation – pot added in, stainless steel material w/ radiation defined, inner surface split halfway, “real wall” defined on the lower half/bottom of the pot with a 100degC temperature (boiling water). Pan temperatures are elevated further past empirical measurements – I’m assuming this is because the pot is holding heat from the flame closer to the pan than without the pot, but I was expecting to see somewhat of a heat reduction due to the pot acting as a sink. Is using a “real wall” adequate considering I don’t really care about what is going on inside the pot?

 

Fourth simulation – fluid (water) subdomain added in to the pot as the volume in the lower half. The subdomain’s temperature (thermodynamic parameter) is defined as 100degC. The same heat source is used. No boundary conditions are applied. The result – all surfaces are now 100degC. It seems like this new fluid subdomain is acting like an infinite heat sink at 100degC. How should I properly set this up?

 

Fifth simulation – Same as the fourth, but with the fluid subdomain temperature set to 25degC. All surfaces are now 25degC, confirming that the fluid subdomain functions as an infinite sink.

 

Sixth simulation – Same as fourth, except that the heat source is now defined as constant 1300degC temperature. Water and pan temperatures now rise. Water/pot elevates to 600 degC (not realistic of course), pan temperature is ~25% less than empirical. Interesting that we are getting closer on the pan temperature front, but given the water temperature, these results can’t be trusted.

 

Seventh simulation – Same as sixth, but transient analysis. Based on a quick boil water test, I know that the water should hit ~124degC in 5 minutes (300s). I stopped at 30s to check in, I am seeing water regions hitting 500degC, so this isn’t working out well either. Pan temperatures are, not surprisingly, low.

 

Questions (collected from above):

 

1) How is heat lost from a heat generating source? I'm assuming it can be shed to the environment - not just the solids defined.

2) Do I need to define the flame as a heat source with a fixed temperature? Or is the heat source itself sufficient? Should I include radiation properties on the surface of the flame “solid”?

3) In many (all?) of my simulations, I do get an error that T>Tmax for the real gas parameters (air) – should I be concerned?

4) How should I properly set up the fluid subdomain such that a heat generation source actually heats it? Based on my results, it seems like only a temperature source will heat the fluid subdomain.

5) Can I set the fluid subdomain to never exceed 100degC (to more accurately simulate boiling water)?

 

Any help appreciated - thanks!

 

-Chris

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