Wen-Jenn Sheu

A comparative study of compact enhanced fin-and-tube heat exchangers

Experimental study on the airside performance of compact slit fin-and-tube heat exchangers was carried out. Test results indicated the airside performance for compact slit fin geometry is relatively independent of the number of tube row and fin pitch. The results are quite different from previous studies concerning the slit fin geometry. This may be attributable to a smaller slit breadth of this study. Test results of the present slit fin were compared with louver fin and plain fin using various comparison methods. In all cases, the relevant airside performance of the interrupted surfaces (including louver and slit) is superior to that of plain fin surface. However, it should be pointed out that the associated superiority may decrease with decreasing fin pitch and Reynolds number. An updated correlation of the airside performance for the slit fin geometry describing 56 test samples is proposed. The mean deviation of the proposed heat transfer correlation is 7.26% and is 7.18% for frictional correlation.

A comparison of the airside performance of the fin-and-tube heat exchangers in wet conditions;With and without hydrophilic coating

An experimental study was performed on compact fin-and-tube heat exchangers in wet conditions. Airside performance for both hydrophilic coated and un-coated surface is examined. It is found that the effect of inlet relative humidity on the heat transfer performance is small. For un-coated surfaces, the effect of inlet relative humidity has a pronounced effect on pressure drops. It is likely that this phenomenon is related to the condensate flow pattern along the fin surface. The heat transfer performance for the hydrophilic coating surface is lower than the corresponding un-coated surface tested at the same wet condition. Further, the degradation of heat transfer performance may be up to 20% for fin pitches of 1.2 mm. The pressure drops for the hydrophilic coated surface are also lower than the corresponding un-coated surfaces. A maximum 40% reduction is observed for plain fin geometry. The effect of inlet condition on frictional performance is more pronounced in the enhanced slit geometry.

AIRSIDE PERFORMANCE OF STAGGERED TUBE BUNDLE HAVING SHALLOW TUBE ROWS

Abstract Airside performance for tube bank having N = 1 ˜6 are reported in this study. Test results indicated that the heat transfer coefficients for N = 1 and N = 2 are comparable and the heat transfer coefficients increase with the number of tube row. For the same P t, P t the heat transfer coefficients for the tube bank containing smaller diameter tube are higher than those of larger tube diameter for N≥2 and is comparable for N = 1. Comparisons of the heat transfer performance were made with several correlations. It is found that the Gnielinski correlation gives the best predictive ability. For multiple-row heat exchangers, the predictive ability of the well-known Žukauskas correlation is also good. But the correlation under-estimate the present I-row heat exchangers. As a consequence, a slight modification to the Žukauskas correlation is proposed that can extend its applicability in N = 1 and ReD < 1000. In addition, modification to the frictional performance of Žukauskas correlation is also made. The mean deviation of the proposed frictional correlation is 11.2%.

Some Observations of the Frost Formation in Fin Arrays

An experimental study concerning the frost formation and the airside performance of a fin array is reported. Visual observation indicates that the frost thickness at the fin base is thicker than that at the tip. During the period of frost formation along the fin array, one can see an apparent boundary separating a region of very thin frost thickness (the region does not show appreciable frost formation) and a region with appreciable frost thickness. Furthermore, the frost-separating boundary is not horizontal but inclined toward the airflow direction. In the entrance region, the frost formation is apparently less pronounced than that at the rear region. If part of the fin surface is above the freezing point temperature but is still below the dew point temperature, the moist air condenses on the upper part of the fin while frost is formed nearby the fin base. The water condensate on the upper part of the fin array may fall off to wash away the porous structure to become a dot-like pattern. With a fixed frontal velocity, the heat transfer coefficient for a fin spacing of 3 mm is slightly increased with the elapse of time and is higher than that of 8 mm fin spacing. The frost thickness increases with time but decreases with the frontal velocity.

Wave-Like Pseudo-Spin Valve Thin Film as a Biosensor

A magnetic biosensor using an anisotropic magnetoresistance effect based on a wave-like pseudo-spin valve is proposed. The wave-like structure formed trenches for attaching cells, which contain Fe3O4 magnetic nanoparticles by endocytosis, and subsequently altered the magnetic properties of pseudo-spin valve (10 nm Ti/8 nm CoFe/[4 nm Cu/4 nm CoFe]4/[4 nm Cu/8 nm CoFe]4/10 nm Ti). We measured the magnetic hysteresis loops via vibrating sample magnetometer (VSM) in three orthogonal directions. It was observed that devices with attaching cells show significant increases of the switching fields compared to the case without attaching cells when the external magnetic field is in parallel to the elongated trenches, considering an easy axis.