الفهرس 
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Abstract
In this work the effect of fin spacing and type of working fluid on the performance characteristics of a counter flow heat pipe-heat exchanger is investigated.
A test apparatus is designed and constructed to achieve heat transfer measurements from six tested fin spacing resulted from externally aluminum finned gravity - assisted heat pipes.
These pipes are arranged in staggered form constituting three rows (6-5-6) R22, R12 and Acetone are used as the working fluids for these pipes
The evaporator and condenser sections of the heat transfer elements are facing the counter - flowing air streams at different operating temperateres.
This atudy is first carried out on bare pipe and finally on the exernally finned pipes usinggggg a 0.3mm aluminum sheet fins with 2mm, 4mm, 6mm, 8mm and 10mm fin spacing.
A detailed analysis of the experimental results ensures that both the heat pipe-heat exchanger effectiveness and the (NTU) is greatly affected by the number of fins in one centimeter.
Also , the overall heat transfer coefficient and the overall conductance of the heat exchanger showed a significant dependence on the heat pipes arragement.
The experimental results showed that:-
1-The heat pipe-heat exchanger effectiveness increases with the decrease in fin spacing while keeping the evaporator temperature constant, an improvement with 5 fin/cm of four times over that of bare pipe.
2-The (NTU) is directly proportional to the number of fins in one centimeter.
3-The overall heat transfer coefficient of the heat exchanger increases as the number of fins in one centimter increases.
4-The overall heat transfer coefficient of the unit increases as Reynolds number on the evaporator side (Reh)
increases.
5-The overall conductance (Q/Atm)of the heat exchanger increases as Reynolds number ratio (R*e)increases.
6-The type of the working fluid filling the heat transfer elements, has a pronounced effect on the effectiveness and the capacity of the heat exchanger. Using R22 achieves an improvement in the effectiveness over 150%than Acetone .
7-from the present result, it is advised to use heat exchangers with staggered heat tr ansfer elements equipped with fins of 5 fin/cm to improve its performance characteristics.