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Temperature Pull Down Simulation using Fluent 

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October 27, 2017, 02:31 
Temperature Pull Down Simulation using Fluent

#1 
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Vignesh Lakshmanan
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Hello All,
I am working on estimating the time taken for temperature pull down inside a deep freezer using Fluent.
With Regards Vignesh 

October 31, 2017, 00:50 

#2 
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Lucky
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Maybe it is related to how you evaluate when the entire refrigerator has reached 20°C.
Can you check, using the htc obtained from the simulation perhaps, what the time constant is using a lumped capacitance approach? Probably this number, after you compute it, will be very close to 115s like you have numerically or it will be much closer to the experimental 2 hrs. My guess is it will be much closer to 2 hrs for the next reason... What is the size of your refrigerator? The thermal penetration dept, even if you assume a type 1 semifinite solid with fixed surface temperature will be very small since the thermal diffusivity of air is 10^6 m^2/s. That is, in 2s, it is unlikely the entire refrigerator has responded to the 18°C boundary condition. Regarding #7. I have a similar concern. I don't have a good feel for what a typical timescale is for the condenser to reach 18 °C. However, HVAC systems are superefficient when the delta T is small which seems to counter the argument. That is, they are very quick at reducing the temperature a small amount initially but much slower to lower the temperature further (a sort of exponential decay or diminishing returns scenario). 

October 31, 2017, 05:34 

#3  
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Vignesh Lakshmanan
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Dear Lucky Tran,
Thanks for your reply. How am I supposed to do this? Quote:
As you said, I am getting this exponential behavior of temperature vs time and Heat flux vs time . Physics wise, I think the model is correct. But I am not sure about the result 

November 1, 2017, 10:25 

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I doubt that a freezer can go from 43C to 18C in 115 seconds... Air does not conduct heat fast at all, so two hours definitly makes more sense to me. As for the wall temperature, it will be so fast compared to the cooling of air that you could get away with neglecting it I think.


November 6, 2017, 05:16 

#5  
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Vignesh Lakshmanan
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Quote:
What I meant was, what HTC to use? That of the steady state or transient run? In both the cases, the HTC at the end of simulation (ie, when the transient run has reached steady state) is of the order of 10^11. Which value should I use 

November 6, 2017, 05:18 

#6  
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Vignesh Lakshmanan
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Quote:
In case I neglect it, already the wall will be having non uniform temperature distribution, wouldnt it be too much approximation? This might increase the error in my results right 

November 6, 2017, 05:23 

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Yes it will be more of an approximation. It just depends on how accurate you results have to be.


November 6, 2017, 05:30 

#8 
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Vignesh Lakshmanan
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November 23, 2017, 01:10 

#9 
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Vignesh Lakshmanan
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Hi All,
I have managed to finally simulate the problem . Next part is how to solve the problem. Using natural convection, the time taken for pull down is too long (1.5 hrs). In order to enhance the HT , it is decided to include a fan inside the domain (Refer attachment), so that it will help in improving the rate of convection . But the problem is how to simulate this one? I tried steady state MRF + Natural convection together, no use.If i need to use moving mesh for fan, the time step is too small to run the natural convection. Kindly give me some inputs regarding this. Regards Vignesh 

November 23, 2017, 11:20 

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Quote:


November 24, 2017, 00:39 

#11  
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Vignesh Lakshmanan
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Quote:
I have planned to run the fan seperately and compute the air flow rate, which then can be given as velocity input, such that the fan sucks from and delivers within the deep freezer. As you said, I can calculate the theoretical speed, but why should I adjust cp accordingly? I am modelling natural convection, so that cp is calculated according to density changes right? 

November 27, 2017, 07:30 

#12  
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Quote:
In your case, you cannot alter the Massflow since you can't use/simulate a fan. To still change the Heat transfer, you could change the Cp value by looking at the mass flow increase. So calculate the theoretical massflow, let's say it increases by a factor of two. To simulate the new heat transfer, multiply the Cp of air by the massflow factor, two. See where I'm going? 

November 28, 2017, 00:34 

#13  
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Vignesh Lakshmanan
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Quote:
But I was able to simulate fan and natural convection together !!! There was considerable increase in heat transfer !!! Regards Vignesh 

November 28, 2017, 04:39 

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February 23, 2020, 16:02 

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Hello ViLaks,
I was reading over your thread and I have a problem where I need to achieve a desired temperature inside of a room and was wondering if you were able to achieve the desired internal temperature of 18 C and how much of the air inside was changed to 18 C. I have a few questions: 1. How did you specify your initial conditions to be 43 C? 2. How did you incorporate the fan condition? 3. From later in the thread it was discussed to alter the cp of air, is this what you did as well? 4. Would you be able to lay out what boundary conditions you used (inlet, outlet, fan, turbulence, etc.) Thanks so much. Quote:


February 24, 2020, 00:53 

#16 
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Vignesh Lakshmanan
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Hi Bran,
I used ANSYS Fluent for simulations. To specify initial conditions, I just initialised the problem at 43 °C. I did not proceed with implementing fan for various reasons. I just used a higher capacity compressor and a higher evaporator length. But to answer your question, I used "fan" boundary condition in fluent to simulate fan in steady state. Upon convergence, I switched off fan and switched on Energy equation and transient formulation. Since my problem is inside a closed domain, I did not have any inlet and outlet boundary conditions. Regards VL 

Tags 
fluent, natural convection, pulldown 
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