Page 16 RAIN May 1979 LaSt issue, Phil introdl4ced us to some 0/ the magic tricks he bas observed clJllvectivll air ClIrre11ls perf orming in passive .~oltlr bOllses cw d how their tmderstal1ding opens lip a whole /lew lPrritory ofpassive sola?' design. Here he la,Ys out some how-to principles and techniques for seeing how your own house operates. Soon we may fi11d cadres bearing lighted in' cense sticks 'tracking down everywhere the elusive invisible airs ofcomfort and djstr(!ss. This paper is in "ISES '78", a collection ofpapers from last year's gathering due to be released tbis month. Contact AS of ISES, c/o American Technical University, P.O. Box 1416, Killeen, TX 76541. Phil can be contacted at Bo.'>: 18123, Dell'ver, CO 80218. - TB by Philip Henshaw Convection, or thermal air motion, is a rich cyclic behavior and a delicate measure of house d imate. While not simple, convection is very orderly and readily observable using simple rools. Pursuit of house climate understanding involves some largely unorthodox procedures. The heart of my study technique involves the intensive personal observation of single 34-hour periods. My equipment includes a 24-channeJ chart recorder, lots of thermocouples, half a dozen 'hot wire' anemometers, a couple of pyronometers, and then, very importantly, incensc ticks and a refined attention ro skin sensation. Smoke trails have become my best scientific tool. The Importance of Air Flow Convection is a large heat mover and a very delicate indicator of the balance of remote surface temperatures, building geometries, material properties and outside influences. The complex path of a unit of energy into and then out of a home usually includes travel by means of natural air circulation. The thermal action, of solar homes especially, involves repeated internal energy flows between different parts. These radiant, conductive and convective flows are only readily observable by studying convection. The scales of convection links in the energy flow path can be seen in thc following normal case calculation: Through an open doorway to a room , in any .house in winter, there might well bc found a two-foot-deep cool air stream at the bottom, and its counterpart warm air stream at the top. A normal temperature difference of 3 degrees F. and a normal flow rate of 2 ft. per sec. yields a one-way mass flow rate of 3000 lb. of air per hour, and a heat flow rate of 2200 BTU per hour. That is 400 BTU per sq. ft. hour! If one observed that this flow was the average of the day, one might conclude that the 15 cents a day in energy flow might be saved by closing the door. However, because in each situation, with the door open and with it closed, different energy flow dynamics are established, it is not likely that your predictio!l would be accurate. CONVECTION OBSERVATION FOR NATURAL CLIMATE DESIGN In the case of convection on a direct gain mass wall, a significant fraction , say a third, of the incident light energy is immediately carried off by convection. At night a simila~ fraction of the 'stored' energy is carried off the same surface by convection. It is impon ant to truly understand where this energy goes. Quite often both these day and night currents directly supply the cold window down drafts, thus exaggerating the heat loss. This is not necessary. Principles of Order With convection being such a large volume business, one would perhaps expect, but does not usually find, a large amount of turbulence. Convection is a startlingly orderly process. The natural response cycles generate discrete air currents which deftly avoid disturbing each other's paths. When one does block another's path, the other usually waits until the one is finished. The following arc seven principles of order whic-h seem to be in operation for thermal air currents. ]. Because thermal air currents are the interaction of fluids of different temperature and density, they don't mix well. 2. Thermal air currents often slide along.effortlessly, within very sharply defined mid-air boundaries. I am a bit suspicious of how a low friction boundary surface can exist in mid-air, but I've observed them time and again. The magic offrictionless boundaries between currents seems to be that the air at the boundary is not in motion. 3. T~",u,lIy , lug' "ill ,;, 'p,eo b"wccn ,;"-, unen". .~(~~
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