الفهرس 
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Abstract
an experimental study of amulti tubes helically coiled heat exchanger was performed under constant heat flux boundary conditions. the annulus fluid used in this was water. to simply maintain and control constant heat flux boundary conditions on the inner tubes surfaces, electrical rod heaters are used to simulate the inner tubes of the multi tubes-in-tube heat exchanger. the study aims to investigate the effects of some of the geometric parameters of the multi tubes in tube heat exchanger, such as the number of the inner tubes and the annulus hydraulic diameter,on the performance of the heat exchanger. different test sections with different numbers of inner rod heaters were designed, manufactured and tested. four multi tubes in tube heat exchangers with one, three, four and five inner rod heaters were tested in this work.the experiments covered awide range of reynolds number, from about 350 to about 6500. this range covers the laminar range of fluid flow inside helical tubes. moreever, the investigation was conducted for awide range of input heat rates, from about 523 W to about 3300W.the test section consisted of multi tubes inserted inside an outer tube shell. the groub was coiled in the form of a helical coil. the outer tube shell of the heilcal coil was copper tube of 23 MM and 25MM inside and outside diameter, respectively. the inner tubes were simulated by 6MM diameter electrical rod heaters. the rod heaters were constructed from stainless steel-316 thin tubes with nickle chrome resistance wire imbedded inside it and the heater was electrically insulated with aspecial powder from the outher shield.