Experimental and numerical investigations of the performance of three concentric pipes heat exchanger
Saqab, S. Jarallah
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The three concentric pipes heat exchanger is a slightly modified version of double tube heat exchanger. Although the heat exchanger designs have shown extensive progress, they are generally limited to few of many possible flow arrangements and mostly restricted on two fluid heat exchangers. A three concentric pipes heat exchanger is fabricated wherein three fluids, namely hot water, cold water, and normal water flow with different temperatures and also with different mass flow rates. Experiments were conducted for different mass flow rates of the hot, cold, and normal fluids for co-current and counter-current flow arrangements under insulated and non-insulated conditions of the heat exchanger. Two flow combinations for the fluids are taken, first when the cold water flows through the outer annulus, and the normal water flows through the inner pipe, and the second when the cold water flows through the inner pipe and normal water flows through the outer annulus, by allowing the hot water to flow through the inner annulus in both combinations. It is found that the temperature variation in the first combination is better than the second one where the drop in outlet temperature of the hot water is higher. Finite element method is used to predict the temperature variation of the three fluids along the length of heat exchanger by developing a computer program using MATLB software. It is found that the numerical predictions of the temperature variation of the three fluids by using the finite element method follow closely to those obtained from experiments both in magnitude and trend. The analytical expression available in the literature to predict the crossover point in terms of its location is found to be satisfactory in the present investigation. Finally, from the parametric analysis of the insulated heat exchange, it is found that the 1 R and NTu affect strongly on the thermal performance ,Similarly, from the parametric analysis carried out for the noninsulated heat exchanger, it is found that there is a pronounced effect of ¥ NTu on the temperature variation of the three fluids especially on the outlet temperature of cold water for the range of the surrounding ambient temperature considered in the present analysis. For ¥ NTu 0.05, the percentage change in outlet cold water temperature is found to be 12.42% when the dimensionless ambient temperature varied from -0.25 to 0.5. This percentage change increases to 23.29% when ¥ NTu is further increased to 0.1 while, for other design parameters, the percentage change in temperatures are nearly constant.