INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res., 22, 1199 — 1204 (1998) THE EFFECT OF VISCOUS DISSIPATION ON THE PERFORMANCE OF A RECUPERATOR AHMET Z. SAHIN* Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia SUMMARY An analytical study is performed to demonstrate the effect of viscous dissipation on the performance of a counter-flow recuperator. Four liquids, namely, water, transformer oil, engine oil, and glycerol are selected as working fluids. It is shown that a considerable decrease in performance is obtained when the viscous frictional heating is considered. The optimum size of recuperator is found to be finite and depends on the thermal properties of the running fluids. The effect of viscous heating on the temperature variation in the recuperator is also discussed.  1998 John Wiley & Sons Ltd. KEY WORDS effectiveness; optimum recuperator size; recuperator; performance of recuperator; viscous dissi- pation 1. INTRODUCTION Recuperators are used in a variety of applications such as the petrochemical, power, aerospace, automotive, refrigeration, air conditioning, cryogenics, food, and manufacturing industries. A major part of the thermal energy used in the industry is transformed in various heat exchanging devices. An improvement of only a few percent in energy efficiency of thermal process may yield a considerable amount of fuel saving. The energy efficiency of a recuperator can be improved by enhancement of heat transfer. However, both heat transfer and recuperator efficiency depend on the flow pattern resulting from the design and enhancement technique and the viscous friction within the recuperator. In the process of designing a heat recuperator, there are two main considerations. These are the heat transfer rates between the fluids and the pumping power requirement to overcome the fluid friction and move the fluids through the recuperator. Although the effect of viscous dissipation is negligible for low-velocity gas flows, it is important for high-velocity gas flows and liquids even at very moderate velocities (Shah, 1981; Kays and Crawford, 1993). Heat transfer rates and pumping power requirements can become comparable especially for gas-to-gas exchangers in which considerably large surface areas are required when compared with liquid-to-liquid heat exchangers such as condensers and evaporators. In addition, the mechanical energy spent as pumping power to overcome the fluid friction is worth 4 to 10 times its equivalent heat (Kays and London, 1984). Therefore, viscous friction which is the primary responsible cause for the pressure drop and pumping power requirements is an important consideration in the heat exchanger design. Viscous dissipation and flow work in ducts have been studied by many investigators (e.g. Brinkman, 1951; Ou and Cheng, 1973; Ou and Cheng, 1974; Shah, 1981). Brinkman (1951) and Ou and Cheng (1974) studied the effect of viscous dissipation on thermal entrance heat transfer in pipe flows with uniform wall temper- ature. The effect of viscous dissipation on the thermal entrance heat transfer in ducts with constant wall heat flux has been investigated by Brinkman (1951), and Ou and Cheng (1973) with the assumption of constant *Correspondence to: Dr A. Z. Sahin, Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia CCC 0363-907X/98/131199— 06$17.50 Received 5 June 1998  1998 John Wiley & Sons, Ltd. Accepted 12 June 1998