Yu-Juei Chang

A generalized heat transfer correlation for Iouver fin geometry

A generalized heat transfer correlation for louver fin geometry is developed with the aid of a large data bank. This data bank consists of 91 samples of louvered fin heat exchangers with different geometrical parameters, including louver angle, tube width, louver length, louver pitch, fin length and fin pitch. For the corrugated louver fin geometry, it is shown that 89.3% of the corrugated louver fin data are correlated within ± 15% with mean deviation of 7.55%. The inclusion of the plate-and-tube louver fin data in the heat transfer correlation (equation (Al)) results in a mean deviation of 8.21%.

Sensible heat and friction characteristics of plate fin-and-tube heat exchangers having plane fins

Abstract In the present study, 15 samples of plate fin heat exchangers with different geometrical parameters, including the number of tube rows, fin spacing and fin thickness are tested and compared in an induced flow open wind tunnel. Results are presented in the form of friction factor and Colburn j -factor against Reynolds number based on the tube collar diameter in the range of 300 to 7500. Comparisons with the existing plate fin correlation are also reported. It is found that the fin spacing does not affect the heat-transfer coefficient. The number of tube rows has negligible effect on the friction factor, and the fin thickness does not affect the heat-transfer or friction characteristics.

Performance of Rectangular Fin in Wet Conditions: Visualization and Wet Fin Efficiency

An experimental study concerning the performance of a rectangular fin in both dry and wet condition was carried out. The visual observation of the dehumidifying phenomenon identified four regions, including the fully dry, very fine droplet, larger droplet, and film-like region, Experimental results showed that the effect of dry bulb temperature on the wet fin efficiency is very small. The dry fin efficiency is about 15-25 percent higher than that of the corresponding wet fin efficiency. For fully wet condition, the effect of relative humidity on the fully wet fin efficiency is also small. For partially wet surface, a considerable influence of the relative humidity on the fin efficiency is encountered. Test results for the fully wet fin efficiency agree well with some of the previous studies but disagree with some of the previous investigations. The main cause to this controversy may be attributed to the formulation of the relation between the humidity ratio and the fin temperature. The fully wet fin efficiency decrease slightly with increase of fin base temperature, However, the effect of fin base temperature on the dry fin efficiency is relatively small.

Effect of circuit arrangement on the performance of air-cooled condensers

Abstract An experimental study was carried out to investigate the effect of circuitry on the performance of wavy finned condensers. A total of eight arrangements were made and tested. The arrangements included six 1-circuit and two 2-circuit arrangements. For the one-circuit arrangement, the test results indicate that counter-cross flow would give better performance than other arrangements. However, heat conduction along the fins may offset the benefits of the counter-cross arrangement. This study has proposed two modifications to the counter-cross flow arrangement. For the two-circuit arrangement, a unique characteristic of “pressure gain” was observed when one circuit is completely condensed and the other is still in two-phase region.

Two-Phase Flow Resistance of Refrigerants R-22, R-410A and R-407C in Small Diameter Tubes

The two-phase frictional characteristics of refrigerants R-22, R-407C, and R-410A in small diameter tubes are reported. Tests were performed in 3, 5, 7, and 9 mm diameter tubes. The range of mass velocities ranges from 50 to 700 kg m −2 s −1 . It is found that for a fixed mass flux, saturation temperature and quality, the pressure drops for R-22 are higher than those of R-407C and R-410A. The predictive ability of existing correlations such as the Friedel correlation, the homogeneous model, and the Souza and Pimenta correlation are comparable at a mean deviation of 30 ∼ 35%. However, the predictive ability of the Chisholm correlation is quite poor. Based on the test results of this study and other investigations, modifications to the homogeneous model and the Chishom correlation are made; the relevant mean deviations of the modified correlations are around 17 ∼ 18%.

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.

Frictional Pressure Drops of Two-Phase R-134A Flow in Horizontal and Vertical U-Type Wavy and Straight Tubes

This study presents the measurements of R-134a two-phase frictional pressure gradient subject to vertical and horizontal arrangements of a U-type wavy tube with inner diameter of 5.07 mm and a curvature ratio of 5. The ratio between two-phase pressure gradients of U-bend and straight tube is about 2.5 - 3.5. For the straight tube, the frictional two-phase pressure gradient ratio between the vertical and horizontal arrangements is marginally higher (1.0 - 1.2) for annular flow pattern at x > 0.5, and is 1.0 - 1.4 for the U-bend in the wavy tube. The higher resistance in the vertical arrangement is due to the buoyancy force against the flow inertia. However, for x < 0.5, this ratio is gradually increased due to the difference of flow pattern. The ratio is increased to 1.8 in the straight tube. For the U-bend, the ratio is 2.1 for flow entering the upper tube and is 1.5 for flow entering the lower tube at x = 0.1 and G = 200 kg/m2 ·s. For the vertical wavy tube, additional effects like the flow pattern transition, liquid flow reversal, and freezing slug may cause additional pressure drops.Copyright

Influence of Vertical Return Bend on the Two-Phase Flow Pattern in 3-mm Minichannel

This study provides a qualitatively visual observation of the two-phase flow patterns for air-water mixtures inside a 3 mm smooth tube with the presence of vertical return bend. The curvature ratio (2R/D) is 3.2 whereas the total mass flux is from 70 to 800 kg/m2 s. The flow can be either entering from the upper of the tube or from the lower tube. However, it is found that there is no great difference between those flow entering at the upper tube and that of the lower tube if the inlet mass flux and vapor quality is the same. For a mass flux of 70 kg/m2 s at a vapor quality to 0.009, as the flow is approaching the return bend, one can observe a fluctuating phenomenon at the tail of the long slug that leads to a liquid ripple around the periphery. When the air slug is trying to penetrate the preceding liquid in the return bend, the shape at the front of the air slug was sharpened. A further increase of the vapor quality to 0.05, the flow after the return bend was temporarily turned from stratified flow into the annular flow. At a higher mass flux of 300 kg/m2 s, unlike those flow pattern at 70 kg/m2 s, the increase of the vapor shear interacts with the centrifugal force and the accumulated liquid within the return bend forces the Taylor bubble to be completely disordered. There is no separating and re-merging phenomenon of the air slug for the slug flow pattern across the return bend even for a very low vapor quality of 0.001. This is quite different from those with larger diameter tube (Chen et al. 2002, Wang et al. 2003b, 2003c).Copyright

Air-water two-phase flow in a 3-mm horizontal tube

Two-phase flow pattern and friction characteristics for air-water flow in a 3.17 mm smooth tube are reported in this study. The range of air-water mass flux is between 50 to 700 kg/m2⋅s and gas quality is between 0.0001 to 0.9. The pressure drop data are analyzed using the concept of the two-phase frictional multipliers and the Martinelli parameter. Experimental data show that the two-phase friction multipliers are strongly related to the flow pattern. Taitel & Dukler flow regime map fails to predict the stratified flow pattern data. Their transition lines between annular-wavy and annular-intermittent give fair agreement with data. A modified correlation from Klimenko and Fyodoros criterion is able to distinguish the annular and stratified data. For two-phase flow in small tubes, the effect of surface tension force should be significantly present as compared to gravitational force. The tested empirical frictional correlations couldn’t predict the pressure drop in small tubes for various working fluids. It...