I have modeled a heat pump and I choose finned type heat exchanger for both condenser and evaporator. I have divided the condenser in four section with around 83mm length of each. I divided the evaporator to 6 segments with a length of 24mm. I used NTU method and performed 100 iteration in condenser and 37 iteration in evaporator. The number of tube are 2 and capacity is around 5kw with a compressor working at 1000 RPM. The outdoor air temperature is 35, so what I am having issue is the outlet temperature of last sections of condenser that are third and fourth always converge from first iteration (T_third_out [1]= 81 ......... T_third_out [100]=35. Its like after 70th iteration the temperature becomes constant at 35 and also Tair outlet from condenser is max 42 degree and the it become constant after 35 iteration with 35 degree.
Now I am confuse how to optimize the heat exchanger with R134a, if I reduce the length the heat exchanger become extremely small and the temperature main same and also if I increase the section length of these heat exchanger the model does not work with 164 cc displacement volume. The model also does not work at more than 1000 RPM.
Its a very long and complex model, I want to know in order to run the model at high capacity what things I need to change and if someone can help why the temperature always converge on 35 degree as I have not specific the condenser outlet.
Please guide me if someone can do ?
It sounds like you're dealing with a challenging optimization problem for your heat pump system. There could be several factors contributing to the convergence issues and the inability to run the model at higher capacities. Here are some steps you can take to troubleshoot and potentially optimize your heat exchanger:
By systematically addressing these aspects of your heat pump system, you can potentially improve its performance, optimize the heat exchanger design, and overcome convergence issues at higher capacities.
If the liquid is converging on the air temperature it implies that the heat exchanger is massively oversized or there is an error in your codes (more likely). The calculated NTUs are probably incorrect. Beware specific heat units: commonly quoted in kJ/kg/K while the NTU needs J/kg/K, I've come across this issue before.
Jack Broughton It is already too small, so the point is it cannot be oversized. But maybe its the iteration which doing the convergence to ambient temperature 35 degree. Its converging from 81 degree to 35 degree.
So I am confused
Impossible to comment on what the error in the program is without information. You clearly have a fault in the program that you are using.
Jack Broughton Yeah I also think the same since the maximum heat transfer is happening in first row and then for 2nd rows it is just showing a minute temperature with almost zero heat transfer