الفهرس 
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Abstract
The winter heating performance of a wickless heat pipe (Thermosyphon) solar collector for out-door in desert climatic conditions in Egypt is presented. The collector consists of three Thermosyphons having identical equal length, 2 (m) and of different outer diameters (OD);
which are 28.6,22.2 & 19 (mm). Three pairs of finned plates of the same
surface area have been attached to the three pipes, to enhance the solar
absorbing surface. Refrigerant Rll has been used as the working fluid.
Temperature distribution profiles at different circumferential sections along
the pipe longitudinal axis have been measured and drawn for each
Thermosyphon to determine the maximum heat capacity. The effects of both the unsteady heat flux at the evaporator section and the intermettent
cooling at the condenser section of the heat pipes on the collector
efficiency have been investigated. The heat capacity and hence thermal
efficiency are found to be in direct proportion to the pipe diameter for the
biggest and middle diameter pipes. A state of functioning failure (burn-
out) has been attained with the smallest diameter Thermosyphon. It is
concluded, also, that the wickless heat pipe solar collector overcomes
most of the disadvantages of the common direct solar collectors.