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Offset strip fins are used in Compact Heat Exchangers. A new technique for the prediction of heat transfer characteristics of these surfaces has been established. The method utilizes an analogy between heat and momentum transfer proposed for short thermally developing laminar flow. The present work exploits the intuition that the analogy can be utilized for any repeated flow structure with heat transfer from surfaces that are also repetitive in array. In the offset strip fin geometry, the original form of this analogy shows a constant bias, which depends on the fin geometry. This leads to the incorporation of the fin geometrical parameters to a new analogy equation resulting in a generalized form of equation for the entire range of offset strip fins. The results clearly indicate that the heat transfer characteristics of offset strip fin surfaces could be predicted by measuring pressure drop across them alone. � 2006 R.T. Edwards, Inc.

The safety issues of liquid metal fast breeder reactors (LMFBR) are crucial due to the fact that a highly reactive and hazardous fluid like liquid sodium is used as coolant. One of the extreme cases, which can occur in a fuel subassembly of an LMFBR, is a total blockage of liquid inside the subassembly, which may lead to boiling of sodium. The present study addresses this problem by conducting experiments on a 19-rod bundle assembly enclosed inside a tall hexagonal enclosure. Liquid sodium is used as the heat transfer fluid. The natural convection mode of heat transfer is the main focus of investigation with a co-flowing air through an annular packed bed to simulate the neighbouring subassemblies. The maximum temperature achieved under different rates of power generations and air flow conditions are observed. Also the radial temperature distributions at different planes under different operating conditions of power and air flow rates have been observed. The results are of significant importance for validating analysis for the purpose of prediction of boiling incipience in an LMFBR subassembly under totally blocked condition. ? 2004 Elsevier B.V. All rights reserved.

Fast breeder nuclear reactors used for power generation, have fuel subassemblies in the form of rod bundles enclosed inside tall hexagonal cavities. Each subassembly can be considered as a porous medium with internal heat generation. A three-dimensional analysis is carried out here to estimate the heat transfer due to natural convection, in such an anisotropic, partially heat generating porous medium, which corresponds to the typical case of blocked flow in a fuel subassembly inside the reactor core. Using the finite volume technique, the temperatures at various locations inside hexagonal cavity are obtained. The simulations by the three-dimensional code developed are compared with the results of experiments [Suresh, Ch.S.Y., Sateesh, G., Das, Sarit K., Venkateshan, S.P., Rajan, M., 2004. Heat transfer from a totally blocked fuel subassembly of a liquid metalfast breeder reactor. Part 1: Experimental investigation. Nucl. Eng. Design, present issue] conducted using liquid sodium as the heat transfer fluid. Further, the code is used to predict the maximum temperature in typical liquid metal fast breeder reactors to find the power level where the liquid sodium starts boiling. It helps to decide the power level for initiation of monitoring the temperature for the purpose of reactor control. ? 2004 Elsevier B.V. All rights reserved.

The present work is aimed at building a comprehensive heat transfer model for pool boiling on horizontal tubes combining sliding bubble mechanism with natural convection, microlayer evaporation and transient conduction during boundary layer reformation, to predict the boiling heat flux for a given wall superheat. The model has been compared to experimental results from literature for R134a and also to independent experiments conducted by the authors for water. Even with a number of simplified assumptions a reasonably good agreement has been observed between model and experiments in the low and medium heat flux region and moderate pressure.

Pool boiling characteristics on horizontal tubes with diameter lying between wires and industrial tubes have been investigated experimentally. Boiling experiments are carried out at near atmospheric pressures with water and R-123 as boiling liquids. The experimental results show quite a different boiling behavior compared to larger tubes or plates. The commonly used correlations are found to be ineffective over this range of diameters. A developing sliding bubble mechanism can be attributed to such behavior which shows a strong diameter effect. The heat transfer is found to increase with diameter which is contrary to that on industrial size tubes. Useful boiling data are presented in this range of diameters in which experimental data are scarce. The need for more investigations is stressed in view of emerging applications in this area.

To predict the transient response of multipass plate heat exchanger, an analysis is presented based on an axial heat dispersion model in the fluid which takes deviation from ideal plug flow into consideration. The method can be utilised to simulate the exit temperatures for both the fluids for any given arbitrary temperature transient at inlets. The methods takes the 'phase-lag' inside the distribution port into consideration and analyse the multipass plate heat exchanger by considering it as an assembly of single pass plate heat exchangers. The solution technique for equal number of passes on both the sides is different from that for unequal number of passes on the two sides. Examples of computation for the response of 1-2 and 2-2 pass configuration have been presented. The 1-2 pass exchanger is solved by analysing successive modules of the heat exchanger. The 2-2 pass heat exchanger is required to be simulated by iterating the responses of two smaller heat exchangers to which it has been divided. The method involves a Laplace transformation with the temporal variable, solution in space domain and numerical inversion of Laplace transform back to the time domain. The results indicate the strong effect of axial dispersion in fluid on the transient response. The effect of number of plates and number of transfer unit has also been studied which brings out a comprehensive picture of responses of a multipass plate heat exchanger in transient regime. (C) 2000 Elsevier Science Ltd. All rights reserved.To predict the transient response of multipass plate heat exchanger, an analysis is presented based on an axial heat dispersion model in the fluid which takes deviation from ideal plug flow into consideration. The method can be utilized to simulate the exit temperatures for both the fluids for any given arbitrary temperature transient at inlets. The methods takes the `phase-lag' inside the distribution port into consideration and analyze the multipass plate heat exchanger by considering it as an assembly of single pass plate heat exchangers. The solution technique for equal number of passes on both the sides is different from that for unequal number of passes on the two sides. Examples of computation for the response of 1-2 and 2-2 pass configuration have been presented. The 1-2 pass exchanger is solved by analyzing successive modules of the heat exchanger. The 2-2 pass heat exchanger is required to be simulated by iterating the responses of two smaller heat exchangers to which it has been divided. The method involves a Laplace transformation with the temporal variable, solution in space domain and numerical inversion of Laplace transform back to the time domain. The results indicate the strong effect of axial dispersion in fluid on the transient response. The effect of number of plates and number of transfer unit has also been studied which brings out a comprehensive picture of responses of a multipass plate heat exchanger in transient regime.

A second law analysis is presented for thermally dispersive flow through a plate heat exchanger. It is well known that in plate or plate fin type heat exchangers the backmixing and other deviations from plug flow contribute significantly to the inefficiency of the heat exchanger, which is of importance to heat exchangers working in the cryogenic regime. The conventional axial heat dispersion model which is used so far is found to be better than 'plug flow' model but still unsatisfactory where the timescale related to heat transfer is comparable with the thermal relaxation time for the propagation of dispersion. The present work therefore considers dispersion as a wave phenomenon propagating with a finite velocity. The study discusses the nature of variation of different contributions to total exergy loss in the heat exchanger with respect to dispersion parameters of the Peclet number and propagation velocity of the dispersive wave. The practical example of the single-pass plate heat exchanger demonstrates how a second law optimization can be carried out for heat transfer equipment under such conditions. ? 1998 Elsevier Science Ltd. All rights reserved.