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Abstract:It is good for the consumer to have solar cookers of various varieties in terms of geometrical designs, performance and price but it is a ch allenge to develop a u niform test standard for evaluating the thermal performance of the cookers irrespective of their geometrical construction. Due to the lack of uniform test protocol, consumer cannot compare the quantitative performance of the cookers of different configuration and become confused. For this end, we plotted graphs between exergy output and temperature difference, for solar cookers of different designs and it resembled a parabolic curve for each design. The peak exergy (vertex of the parabola), can be accepted as a measure of devices' fuel ratings. The ratio of the peak exergy gained to the exergy lost at that instant of time can be considered as the quality factor of the solar cooker. Besides, the exergy lost is found to vary linearly with temperature difference irrespective of the topology of the device and the slope of the straight line obtained through curve fitting represents the heat loss coefficient of the cooker. The proposed parameters can lead to development of unified test protocol for solar cookers of diversified designs.

This study concerns the development of a set of mechanistic criteria capable of predicting the flow conditions for which gravity independent flow condensation heat transfer can be achieved. Using FC-72 as working fluid, a control-volume based annular flow model is solved numerically to provide information regarding the magnitude of different forces acting on the liquid film and identify which forces are dominant for different flow conditions. Separating the influence of body force into two components, one parallel to flow direction and one perpendicular, conclusions drawn from the force term comparison are used to model limiting cases, which are interpreted as transition points for gravity independence. Experimental results for vertical upflow, vertical downflow, and horizontal flow condensation heat transfer coefficients are presented, and show that, for the given test section, mass velocities above 425 kg/m 2 s ensure gravity independent heat transfer. Parametric evaluation of the criteria using different assumed values of mass velocity, orientation, local acceleration, and exit quality show that the criteria obey physically verifiable trends in line with those exhibited by the experimental results. As an extension, the separated flow model is utilized to provide a more sophisticated approach to determining whether a given configuration will perform independent of gravity. Results from the model show good qualitative agreement with experimental results. Additionally, analysis of trends indicate use of the separated flow model captures physics missed by simpler approaches, demonstrating that use of the separated flow model with the gravity independence criteria constitute a powerful predictive tool for engineers concerned with ensuring gravity independent flow condensation heat transfer performance.