Transient Effects of Solar Radiation

May 17, 2022 Troy Baumgardner

Spring has officially arrived promising warmer temperatures and a higher probability of seeing the sun.  The warming glow of the sun’s rays is a welcomed sight this time of year, but in a few short weeks, we will be into the heat of summer when those same rays can add extra heat to struggling air conditioning and ventilation cooling systems.  Rays can pound opaque surfaces, converting solar radiation to surface heat which can transfer through walls and roofs to occupied or unoccupied spaces.  Solar radiation can also pass through windows to heat interior surfaces.  Understanding the impact of solar radiation can be the difference between a functional cooling system design and one which will allow occupants or equipment to overheat.  Luckily, Ansys CFD analysis can help.

If solar radiation is a minimal part of the heat load in a space then the entire surface exposed to radiation may see a, more or less, constant heat load and could be analyzed as a steady-state simulation.  What if, however, the radiation entering through a window is the only source of heat in the room or you are looking at an exterior space that will be shaded for most of the day?  This second case was well shown by a picture my colleague took of his back deck several months ago showing an otherwise dry deck with patches of unmelted snow:

Image1

I was intrigued by this image as I didn’t understand how an even dusting of snow melted and evaporated from most surfaces while leaving a patch due to shading by the handrail.  As a good CFD engineer, I decided to run an analysis to see if I could replicate these results.  A model was built to resemble the deck and I started working on the analysis setup. 

Image2

This analysis would have to account for shadows, unlike the cases where a constant heat load could be applied to all surfaces.  To know where shadows would be the analysis would also have to know where the sun is, and thus the issue.  The sun moves.   Analyzing this model as steady-state, either with radiation coming from one location or applying a heat load to the surfaces, would act as if the sun wasn’t moving.  The results of a steady-state analysis would produce either artificially low or artificially high heat loads.  Sure, a fixed solar load would create the shadows shown in the picture, but it wouldn’t explain how the snow kept from melting hours after sunrise when all surrounding snow was gone.  This analysis would have to be transient with a moving point of origin for the radiation. 

To do this the orientation of the model and path of the sun must be properly identified.  This required consulting with my colleague to determine the location, orientation, and date the image was taken.  As I assumed, the step off the deck roughly faces east, but I thought the picture was of the northeast corner of the deck looking at a shadow from the east railing, as shown by the red circle.  The analysis showed otherwise. 

The analysis starts just before sunrise and calculates the accumulated radiant heat impacting the deck.  The images below show the shaded line on the deck 2 hours, 4 hours, 6 hours, and 8 hours after sunrise:

Image42 Hours After Sunrise

 

Image34 Hours After Sunrise

 

Image66 Hours After Sunrise

 

Image58 Hours After Sunrise

 

This shows the picture is taken, not looking northeast, but southeast.  As the sun tracks from east to west with a southward inclination it produces a line of low radiant heat on the deck surface due to the shadow of the south handrail, not the east.   Outputting tabular data from the deck surface shows the red regions have more than enough radiant heat on them to melt and evaporate two inches of snow, even if the deck surface was solid, and produce the effect shown in the picture.

If you are not getting the whole picture of where solar loading may have an impact on your system, contact Rand Simulation so the CFD team can shed some light on the problem and help you find solutions.  

About the Author

Troy Baumgardner

Simulation Specialist for Fluid and Heat Transfer Applications<br><br> Interests: spending time with my family, cooking, mountain biking, reading

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