external radiant flux input
 

Hi, I wish to simulate the thermal performance of a solar air heater receiving 800 W/m2 of solar radiation. How do I specify this radiation level as input in PHOENICS ? Thanks. cfthan

Re: external radiant flux input
 

It is difficult to answer precisely because there is insufficient information on the details of exactly what you are trying to model. Furthermore, you do not say what PHOENICS Version you are using and what data-input method you are using, i.e. Satellite Menu, Q1 input file or VR Front End. Some general advice is given below.

It depends on how you are modelling the solar heater itself. Assuming that this is a participating solid region, you need a source patch to get the correct amount of energy into the solid.

If the solid edges match the grid lines, an area-type PATCH would normally be used; in the q1 file this could be done by, for instance,

PATCH (name, NORTH, ..... )

COVAL (name, TEM1, FIXFLU, 800) where the solid is on the SOUTH side of the face to which the PATCH relates.

Alternatively, you might choose to distribute the heat directly over the whole of the solid (particularly if it has a complex shape, but you do not intend to model heat transfer within it in detail); for this you would specify a VOLUME-type PATCH, adjusting the VALUE to give the correct total heat flux (which should always be checked in the nett source section of the RESULT file).

Either approach can easily be implemented using the VR Editor: just create a 2-D or 3-D object (depending on the approach being used) and associate a heat source with it by following the options in the VR dialogue boxes.

Re: external radiant flux input
 

Thanks Mike for your quick reply. We are using PHOENICS version 3.1.

(1) The simplest geometry is similar to flow between parallel plates as follows:

Incoming solar radiation

llllllllllllllllllllllllllllllllllllllllllllllllll llllllll

llllllllllllllllllllllllllllllllllllllllllllllllll llllllll

vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv

____________________________________Top plate

--> ambient air in

____________________________________Bottom plate

The top and bottom plates are metallic and may be assumed thermally thin across the depth but temperature and heat fluxes will vary in the flow direction due to heat losses by radiation and convection (eg due to wind) from above and below. Hence both FIXED TEMPERATURE and HEAT FLUX cannot be readily used as boundary conditions for the top and bottom plates when modelling the air flow. Any possibility to use PHOENICS to model both external and internal convection using solar radiant flux, ambient temperature and windspeed as boundary conditions ?

(2) How do I include calculation of radiation exchange between the top and bottom plates in PHOENICS?

(3) For simplicity, we have tried FIXED TEMPERATURE boundary conditions using MEASURED top and bottom plate temperature values. The air stream temperature distribution across and along the flow channel is reasonable in comparison with experimental data. If geometry and flow conditions are changed and no measured plate temperatures are available, what can we do ?

Regards cfthan

Re: external radiant flux input
 

The simplest thing to do is to grid both plates vertically, i.e. represent the plates as participating solids rather than as zero-thickness thin plates. This is more rigorous, and it will make the boundary-condition specification easier when using the radiation model. The radiation can be simulated by use of the pragmatic IMMERSOL method or more accurately by use of the Surface-to-Surface radiation model. I recommend the former as it should be sufficient and it is much easier to use, especially for complex geometries. The model is documented on CHAM's website at: http://www.cham.co.uk/phoenics/d_pol...c/enc_rad3.htm.

Strictly speaking, you should include the boundary-layer flow above the top plate in the solution domain. As a first cut, you could avoid this by specify the convective heat loss from the top plate to the surrounding environment via a heat transfer coefficient. The solar radiative flux should be deposited in the top plate, but unless it is an estimated nett value some heat will be radiated back into the surrounding environment.