RESUMO
Effect of external-recycle operations on the heat-transfer efficiency, specifically for the power-law fluid flowing in double-pass concentric circular heat exchanger under sinusoidal wall fluxes, is investigated theoretically in the developed countries. Given that the fluid is heated twice on both sides of the impermeable sheet, four flow patterns proposed in recycling double-pass operations are expected to make substantial improvements in the performance of heat exchanger device in this study. Theoretical predictions point out that the heat-transfer efficiency increases with the ratio of channel thickness of double-pass concentric circular heat exchanger for all new designs under the same working dimension and the operational condition. The fluid velocity within the double-pass heat exchanger is increased by the fluids flowing through divided subchannels, which contributed to the higher convective heat-transfer efficiency. A simplified mathematical formulation was derived for double-pass concentric circular heat exchangers and would be a significant contribution to analyze heat transfer problems with sinusoidal wall fluxes at boundaries. The results deliver the optimal performance for the proposed four configurations with the use of external recycle compared to those conducted in single-pass, where an impermeable sheet is not inserted. The influences of power-law index and impermeable-sheet position on average Nusselt numbers under various flow patterns are also delineated. The distribution of dimensionless wall temperature was lower at the level of relative smaller thickness of annular channel, and the average Nusselt numbers for four external-recycle configurations and single-pass device were more suitable for operating under same condition. The ratio of the power consumption increment to heat-transfer efficiency enhancement demonstrates the economic feasibility among various configurations of double-pass concentric circular heat exchanger. The results also show that the external-recycle configuration (say Type B in the present study) serves as an important economic advantage in designing concentric circular heat exchangers for heating power-law fluids due to the smaller volumetric flow rate in annular channel with exiting outlet temperature.
RESUMO
An analytical formulation, referred to as conjugated Graetz problems, is developed to predict the temperature distribution and Nusselt numbers for the power-law fluid flowing in a double-pass concentric circular heat exchanger under sinusoidal wall fluxes. A new design of inserting an impermeable sheet into a concentric tube, in parallel, to conduct recycling double-pass operations has been studied theoretically in the fully developed region, resulting in substantial improvements in the performance of heat exchanger device. The analytical solution was derived using the complex functions by transforming the boundary value problem into ordinary differential equations with the aid of the Frobenius method. The influences of power-law index and impermeable-sheet position on average Nusselt numbers with various designs and operating parameters are also delineated. The theoretical predictions show that the heat transfer efficiency is considerably improved through operating the double-pass device compared to via a single-pass circular heat exchanger (where an impermeable sheet is not inserted). The economic feasibility of operating double-pass concentric circular heat exchanger for power-law fluids is exemplified by the ratio of the heat-transfer efficiency enhancement and the increment in power consumption. The double-pass effect from increasing the convective heat-transfer coefficient can compensate for the rise in power consumption, which serves as important economic advantage of this design.
