Chii-Dong Ho

An analytical study of heat and mass transfer through a parallel-plate channel with recycle

Abstract The effects of recycle at the ends on the heat and mass transfer through a parallel-plate channel with uniform wall temperature are studied by an orthogonal expansion technique. The heat and mass transfer problem is solved for fully developed laminar velocity profiles in a parallel-plate channel with ignoring axial conduction, and with fluid properties which are temperature independent. Analytical results show that recycle can enhance the heat transfer rate for high Graetz numbers compared with that in an open conduit (without an impermeable plate inserted and without recycle). The preheating and residence-time are the two effects which are in domination of the heat transfer behavior.

Collector efficiency of double-flow solar air heaters with fins attached

A design for inserting an absorbing plate to divide the air duct into two channels (the upper and the lower) for double-flow operation in solar air heaters with fins attached over and under the absorbing plate has been investigated both experimentally and analytically. The present work is restricted to the case where the outside air is being heated directly, and the configuration investigated here will have lower collector efficiency if the inlet-air temperature is substantially higher than the ambient temperature because of the far greater potential for heat loss from the top. However, the double-flow device introduced here was designed for creating a solar collector with heat-transfer area double between the absorbing plate and heated air. This advantage may compensate for the heat loss from the top when the inlet-air temperature is higher than the ambient temperature. The agreement of the theoretical predictions with those measured values from the experimental results is fairly good. Considerable improvement in collector efficiency of solar air heaters with fins attached is obtained by employing such a double-flow device, instead of using a single-flow example and operating at the same total flow rate. Both the theoretical predictions and experimental results showed that the optimal fraction of airflow rate in upper and lower subchannels is around the value of 0.5. The effect of the flow-rate ratio of the two air streams of flowing over and under the absorbing plate on the enhancement of collector efficiency is also investigated.

Effect of collector aspect ratio on the collector efficiency of upward type baffled solar air heaters

The influence of collector aspect ratio on the collector efficiency of upward type baffled solar air heaters has been investigated theoretically. With constant collector area, the collector efficiency increases with collector aspect ratio. This is the same results as those obtained in the previous work for flat plate solar air heaters without fins and baffles. Although the collector efficiency of baffled solar air heaters is larger than that of flat plate heaters without fins and baffles, the improvement of collector efficiency by increasing the collector aspect ratio is the reverse.

AN ANALYTICAL STUDY ON THE ENRICHMENT OF HEAVY WATER IN THE CONTINUOUS THERMAL-DIFFUSION COLUMN WITH EXTERNAL REFLUXES

A new device of inserting an impermeable sheet or a permeable barrier to divide the thermal-diffusion columns into two subchannels with external refluxes at the ends, resulting in substantial improvement of the separation efficiency of heavy water, has been developed and investigated using orthogonal expansion technique. The analytical results are represented graphically and compared with that in a Clusius–Dickel column of the same size with recycle. Considerable improvement on enrichment of heavy water is obtained by employing such devices with an impermeable sheet or a permeable barrier instead of using the Clusius–Dickel thermal-diffusion column. The effect of sheet or barrier location on the enhancement of the separation efficiency of heavy water has also been discussed.

The improvement of collector efficiency in solar air heaters by simultaneously air flow over and under the absorbing plate

The performance of double-flow type solar air heaters, in which air is flowing simultaneously over and under the absorbing plate, is more effective than that of the devices with only one flow channel over or under the absorbing plate because the heat-transfer area in double-flow systems is double. The effect of the fraction of mass flow rate in the upper or lower flow channel of a double-flow device on collector efficiencies, has also been investigated theoretically and experimentally. Considerable improvement in collector performance is obtained by employing a double-flow type solar air heater, instead of using a single-flow device, if the mass flow rates in both flow channels are kept the same.

Double-pass heat or mass transfer through a parallel-plate channel with recycle

It is the purpose of this work to develop the complete theory with orthogonal expansion techniques and investigate the improvement of heat or mass transfer in a double-pass parallel plate device with external recycle at the ends of conduit.

The influence of recycle on double-pass heat and mass transfer through a parallel-plate device

Abstract The influence of recycle on double-pass parallel-plate heat and mass exchangers with uniform wall temperature has been studied analytically. The heat or mass transfer problem is solved for fully developed laminar velocity profiles in parallel-plate channels by ignoring axial conduction, and with fluid properties of temperature independence. Analytical results show that recycle can effectively enhance the heat or mass transfer rate compared with that in an open conduit (without an impermeable plate barrier inserted for double-pass and recycle operation) . The desirable preheating effect and the undesirable effect of decreasing residence time are the two conflict effects produced by the recycle operation. It was found that the increase in preheating effect by increasing the reflux ratio can generally compensate for the decrease of residence time, leading to improved performance, exceptionally for very small Graetz numbers.

THE OPTIMAL VARIATION OF ZONE LENGTHS IN MULTIPASS ZONE REFINING PROCESSES

The effect of variable zone length on the separation efficiency in zone refining processes was investigated theoretically. During zone refining operation, properly adjusting the zone length along the ingot influences the solute concentration in the liquid zone, leading to an enhancement of the extent of separation. For multipass operations, considerable improvement in separation is obtained by adjusting zone lengths in each pass separately.

The influence of collector aspect ratio on the collector efficiency of baffled solar air heaters

The effect of collector aspect ratio on the collector efficiency of baffled solar air heaters has been investigated theoretically. With constant collector area, the collector efficiency increases when the collector aspect ratio increases. This is the same results as those obtained in the previous work for flat-plate solar air heaters without fins and baffles. Although the collector efficiency of baffled solar air heaters is larger than that of flat-plate heaters without fins and baffles, the improvement of collector efficiency by increasing the collector aspect ratio is reverse.

Improvement in performance on laminar counterflow concentric circular heat exchangers with external refluxes

Considerable improvement of heat transfer in laminar counterflow concentric heat exchangers is obtainable by inserting in parallel an impermeable, resistless sheet to divide an open duct into two subchannels for double-pass operations with external refluxes. Efficiency improvement in heat transfer has been investigated analytically by using an orthogonal expansion technique. The results of improvement in heat transfer efficiency are represented graphically and compared with those in a single-pass operation. The influences of improvement-sheet location and reflux ratio on the enhancement of transfer efficiency as well as on the increment of power consumption have been discussed.