Solar Collector Performance

[Anonymous]

Dear all, I'm designing a simple heat collector which is no more than a wooden box topped by a single glass, inside of the box there is a polystyrene sheet followed by a corrugated zinc roofing sheet. There is a rectangular inlet on one side and an outlet on the other side. Air will flow through the collector and will be heated up by sun radiation during the day (see the image below).

I want to know how much the air will be heated up by passing through the collector, to this I set up a CFD simulation with the following settings:

- Boundary conditions for the inlet: Volume flow rate of 442 ft3/min and for the outlet: Unknown conditions 

- Initial conditions: enclosed air within the collector and all the collector components have a temperature of 13C

- The glass is treated as a Window (solar) but I changed some properties because this a 5 mm single glass

- I set up a transient simulation starting at 8 am running for 9 hours with a time step size of 60, total of 540 timesteps and 32400 seconds simulated, Intelligent solution control is disabled and ADV 5 scheme was applied

- The turbulence method I applied is k-epsilon with Auto startup Off and Tub&Lam ratio of 10000

- I treated the air as an incompressible fluid and checked the heat transfer, auto forced convection and radiation options. For the solar heating, I defined the location and date and added the ambient reference temperature as transient including the local temperature data I have as piecewise linear.

I attached the result for the last time step, showing the temperature profile of the enclosed air, so my questions are:

1. First of all, are these the right settings to run this kind of simulation?

2. Temperatures are reaching more than 300C, I think this is way too high. Am I missing something here?

Your help is highly appreciated

[Jon.Wilde]

Hello!

Here are my thoughts, I hope they are useful.

- Boundary conditions for the inlet: Volume flow rate of 442 ft3/min and for the outlet: Unknown conditions - Not ideal. You need flow rate AND temperature at the inlet and a P=0 at the outlet.

- Initial conditions: enclosed air within the collector and all the collector components have a temperature of 13C - Good

- The glass is treated as a Window (solar) but I changed some properties because this a 5 mm single glass - OK, although you could simply assign a heat flux to it right? Quicker for a first test

- I set up a transient simulation starting at 8 am running for 9 hours with a time step size of 60, total of 540 timesteps and 32400 seconds simulated, Intelligent solution control is disabled and ADV 5 scheme was applied - If you have moving air, I would guess you need a far smaller time step, 1s or so... Or get the air moving with a steady state first and run thermal only as transient, if the air profile is not changing over time

- The turbulence method I applied is k-epsilon with Auto startup Off and Tub&Lam ratio of 10000 - OK as a starting point

- I treated the air as an incompressible fluid and checked the heat transfer, auto forced convection and radiation options. For the solar heating, I defined the location and date and added the ambient reference temperature as transient including the local temperature data I have as piecewise linear. - Do you have natural convection inside? Leave Auto Forced Convection off for sure either way, you need to solve flow and thermal together here

I hope that helps, let us know, we are here to help you.

Thanks,

Jon