International Journal of Scientific Engineering and Technology (ISSN : 2277-1581) Volume No.3 Issue No.7, pp : 899-901 1 July 2014 IJSET@2014 Page 899 An Experimental Investigation of Two Phase Natural Circulation Loop (NCL) with End Heat Exchangers Ummid I. Shaikh 1 , R. R. Kulkarni 2 1 Mechanical Engg. Dept.,Pimpri Chinchwad College of Engineering, Nigdi, Pune, India-411044 2 Mechanical Engg. Dept.,Vishwakarma Institute of Information Technology, Kondhwa, Pune, India-411048 1 ummids@gmail.com , 2 ratnakarkulkarni36@gmail.com Abstract: The steady state behaviour of two-phase natural circulation loop with heat exchangers at the hot and cold ends is studied experimentally. A vertical rectangular and uniform cross section Natural Circulation Loop (NCL) with end heat exchangers is designed and fabricated. Steam is used as hot fluid and tap water as cold fluid. The experimentation is carried out for various hot fluid flow rates. Four flow patterns are identified viz. Small bubbly flow, big bubbles flow, stream of small bubbles, churn flow. It is concluded that Circulation rate of the loop fluid rises with hot fluid flow rate. However heavy fluctuations in the flow rate of coupling fluid are observed the reasons of which are explored. Keywords: Two phase, Natural Circulation Loop, NCL, End Heat Exchangers, coupling fluid. I. Introduction In Natural Circulation Loop the loop fluid flow is driven by thermally generated density gradient. Owing to their simplicity, high heat transfer capability, and passive nature, the principle of natural circulation loops is employed in diverse engineering applications like thermo-syphon boilers, solar, thermal and waste heat recovery systems, nuclear reactors. NCLs involving evaporation and condensation of the working fluid, is specially lucrative due to large density difference of the vapor liquid phase. Numerous investigations on two-phase NCL, both theoretical and experimental, are available in the literature [i to ix]. Various flow patterns were identified in two phase flow viz. Bubbly, slug, churn, wispy-annular, annular [x]. The researchers have studied both steady state and transient performance of the loop .However the review of the literature reveals that the hot and cold ends of the natural circulation loops have been idealized either by constant temperature or constant heat flux conditions. These conditions are not appropriate when the loop exchanges heat with single phase flowing fluids. Particularly, in case of a waste recovery system, energy needs to be transferred from one fluid to another. A natural circulation loop can be conveniently employed for this purpose by incorporating suitable heat exchangers with finite heat capacity fluids at the hot and cold ends. In the present work the steady state behaviour of two-phase natural circulation loop with heat exchangers at the hot and cold ends is studied experimentally. II. The Experimental Set up A rectangular and uniform cross section Natural Circulation Loop (NCL) with end heat exchangers is designed and fabricated. Two heat exchangers of concentric tube are placed along the horizontal top and bottom sections of the loop. Figure.1 Experimental set up of a Two-Phase NCL with end heat exchangers erected for present study The geometrical dimension of the loop are: Loop width – 700mm, Loop height – 1400mm and Loop inner diameter – 12mm and Loop outer diameter – 16mm. The heating/cooling length of concentric tube heat exchanger is 500mm, inner diameter of the shell is 78mm and outer diameter of loop pipe is 16mm. To circumvent the difficulty in inducing pressure drop and thereby changes in the loop characteristics by Incorporation of any differential pressure type flow meter, a calibrated magnetic flow meter (non-intrusive type) is installed in the downcomer to measure the induced loop temporal mass flow rate. The uncertainty involved in measuring the loop flow rate with this instrument is 0.1%. Calibrated thermocouples are inserted at desired locations to measure the temporal variation of temperatures of coupling, hot and cold fluids. A Data Acquisition System (DAS) is used to interface with personal computer (PC) to record the real time data. Steam generated by boiler is used as hot fluid and allowed to pass through the bottom horizontal heat exchanger. Building tap water is used as the cold fluid (heat sink) and allowed to pass through the top horizontal heat exchanger. A condenser is fitted at the outlet of HEHE to condense the