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In the present paper, the main parameters affecting the flow maldistribution have been identified and a detailed analysis has been carried out concerning how the parameters affect the flow maldistribution. Using the thermal model of Rao et al. (2002), the effect of flow maldistribution on the thermal performance of the plate heat exchanger was simulated. The present analysis shows that flow maldistribution needs to be taken into account when the number of channels per fluid is greater than 20 and more particularly for low corrugation angles (? = 30�). Flow maldistribution can be reduced by changing the corrugation angle and then the thermal performance of the large plate package of plate heat exchangers can be improved. � 2007 R.T. Edwards, Inc.

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.

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.

Ablation modeling in cylindrical geometry is a very important problem in the thermal modeling of spacecraft, which has not been dealt with adequately in the literature. In the present work, two-dimensional ablation in cylindrical geometry is considered when the incident heat flux varies axially as well as temporally. A novel idea of using an effective inverse Stefan number, which is analogous to the effective heat of ablation (used in the literature), is proposed to be used as a variable nondimensional latent heat of ablation for ablation modeling, indirectly accounting for various processes involved in ablation. A two-dimensional ablation problem is solved by an alternating direction implicit and adjustable time-step scheme, coupled with boundary immobilization. Quasi-one-dimensional and two-dimensional modeling methods have been compared for two different materials, and the effects of variation of the proposed inverse Stefan number with the incident heat flux and the presence of highly conducting structure bonded to the ablative material have been studied. An optimum modeling method under specific conditions is suggested.

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.