Ing Youn Chen

An empirical correlation for two-phase frictional performance in small diameter tubes

Experimental two-phase pressure drop data in small diameter tubes (D<10 mm) have been collected and updated from the literature which contain eight refrigerant and three air–water datasets. Comparisons between the data and the predictions indicate that the Chisholm correlation fails to predict the data. The Friedel correlation and Souza and Pimentas correlation give fair predictions for the refrigerant data, but fail to predict the air–water data due to the surface tension effect. The homogeneous model shows a better predictive ability (a mean deviation of 34.7%) than the other empirical correlations. In this regard, an empirical correlation based on the homogeneous model was developed. By introducing the Bond number and Weber number to the modified correlation, the new correlation gives a mean deviation of 19.1% based on 1484 data points.

Frictional performance of R-22 and R-410A inside a 5.0 mm wavy diameter tube

The influence of return bend on the frictional performance of R-410A and R-22 in a 5-mm diameter tube is examined with a curvature ratio of 6.63. The existing single-phase correlations give fairly good agreements with the present single-phase data, but the existing two-phase correlations of the return bend fail to predict the present twophase data. For test results of the two-phase flow at G P 200 kg m � 2 s � 1 , ððdP =dzÞc=ðdP =dzÞsÞ is approximately equal to 1.8 and is relatively independent of the vapor quality x. However, at a smaller mass flux of 100 kg m � 2 s � 1 , ððdP =dzÞc=ðdP =dzÞsÞ decreases with x, reaching approximately 5 for x ¼ 0:1. The significant increase of this ratio for G increased from 100 to 200 kg m � 2 s � 1 may be attributed to the change of the two-phase flow pattern.

A comparative study of nozzle/diffuser micropumps with novel valves.

This study conducts an experimental study concerning the improvement of nozzle/diffuser micropump design using some novel no-moving-part valves. A total of three micropumps, including two enhancement structures having two-fin or obstacle structure and one conventional micro nozzle/diffuser design, are made and tested in this study. It is found that dramatic increase of the pressure drops across the designed micro nozzles/diffusers are seen when the obstacle or fin structure is added. The resultant maximum flow rates are 47.07 mm3/s and 53.39 mm3/s, respectively, for the conventional micro nozzle/diffuser and the added two-fin structure in micro nozzle/diffuser operated at a frequency of 400 Hz. Yet the mass flow rate for two-fin design surpasses that of conventional one when the frequency is below 425 Hz but the trend is reversed with a further increase of frequency. This is because the maximum efficiency ratio improvement for added two-fin is appreciably higher than the other design at a lower operating frequency. In the meantime, despite the efficiency ratio of the obstacle structure also reveals a similar trend as that of two-fin design, its significant pressure drop (flow resistance) had offset its superiority at low operating frequency, thereby leading to a lesser flow rate throughout the test range.

Two-Phase Pressure Drop of Air-Water in Small Horizontal Tubes

Two-phase e ow pressure drops of air ‐water in small horizontal smooth tubes of 1.02, 3.17, 5.05, and 7.02 mm are reported. The data are compared with homogeneous, slug, and annular e ow models, as well as the commonly used empirical correlations. The tested results indicate that the empirical correlations available in the literature failed to predictthedata.Theslug e owand annulare owmodelsgivefairpredictionswiththeircorresponding e ow regime data. The homogeneous model shows better predictive ability as compared to other empirical correlations. However, overpredictions of homogeneous and annular models are observed for most of the 1.02- and 3.17-mm data. Thus, modie cationsofthehomogeneousmodeland theFriedel correlation are made by introducing the Bond number, Webernumber, and othersignie cantdimensionless groupsto correlatethetestdata. Themodie ed Friedel correlations and the modie ed homogeneous model show good agreement with the present data, as well as with e ve available data sets of ammonia and refrigerants from the literature.

Two-Phase frictional pressure drop correlations for small tubes

Frictional pressure drop for two-phase air-water and R-410A were measured in small horizontal tubes of 1.02, 3.17 and 5.05 mm diameters. The tested results indicated that both the Chisholm correlation and the Friedel correlation failed to predict the data. Although the homogeneous model gives best predictions as compared to the Chisholm correlation and the Friedel correlation, over-predictions of air-water and underpredictions of R-410A are observed at higher mass flux and higher quality. It is likely that this deviation is related to the effects of surface tension and other physical properties. By introducing the Bond number and Weber number, the homogeneous flow model and the Friedel correlation are modified to correlate the test data. From the comparison, the predictive ability has been improved but it is imperative that further investigations should be made. This is because the present study only examines two kinds of working fluid, namely air-water and R-410A.

Experimental investigation of high performance thermal module with dimple vortex generators

Pressure drop and heat transfer characteristics of different dimple vortex generators arrangement are examined in this study. A total of five heat sinks were made and tested, including plain fin, dimple, two-group dimple, oblique dimple gap 4–12 fin, oblique dimple gap 6–12 fin. The tested results indicate that the pressure drop for dimple fin geometry is significantly higher than other fin types, followed by the two group dimple fin. The fin structure with oblique dimple shows slightly increase of pressure drop as compared to the plain fin surface. The results show that more complicated fin structure will lead to higher pressure drop. The observed IR image of the temperature measurement of the test fin configurations also proves the measurements. The results suggest that the fin with dimple vortex generators is more beneficial than that of plain fin geometry. The oblique dimple fin is especially useful for air-cooling applicable for electronic devices to achieve effective augmentations without suffering from significant pressure penalty.

Liquid flow distribution in compact parallel flow heat exchangers

This study experimentally and numerically investigates the single-phase flow into parallel flow heat exchangers with inlet and outlet rectangular headers having square cross section and 9 circular tubes. The effects of inlet flow condition, tube diameter header size, area ratio, flow directions (Z and U type), as well as the gravity are investigated. The experimental results indicate that flow distribution for U type flow is more uniform than Z type flow. Depending on the inlet volumetric flow rate, the flow ratio at the first several tubes can be lower than 50% of the last tube for Z type arrangement, and this phenomenon becomes more and more pronounced with the rising velocity at the intake conduit. The mal-distribution can be eased via reducing the branching tube size or increasing the entrance settling distance at the intake conduit. It is found that the influence of gravity on mal-distribution is negligible and the mal-distribution is associated with the jet flow pattern. The numerical result also shows a small eddy flow formed at the inlet of the first tube by the nearby vortex circulation which would reduce the flow rate to the front tubes.

Experimental Studies of Nozzle/Diffuser Micropumps Using Enhancement Structure

This study conducts an experimental study concerning the improvement of nozzle/diffuser micropump design subject to enhanced structures. A total of three micropumps, including two enhancement structurs having two-fin or obstacle structure and one conventional micro nozzle/diffuser design. It is found that the pressure drops across the designed micro nozzles/diffusers are increased considerably when the obstacle or fin structure are added. The resultant maximum flow rates are 42.08 mm3 /s and 50.15 mm3 /s for conventional micro nozzle/diffuser and added two-fin structure in micro nozzle/diffuser operated at a frequency of 350 Hz. It is found that the mass flowrate for two-fin design surpasses that of conventional one when the frequency is below 400 Hz but the trend is reversed with a further increase of frequency. This is because the maximum efficiency ratio improvement for added two-fin is appreciably higher than the other design at a lower operating frequency. In the meantime, despite the efficiency ratio of the obstacle structure also reveals a similar trend as that of two-fin design, its significant pressure drop (flow resistance) had offset its superiority at low operating frequency, thereby leading to a least flowrate throughout the test range.Copyright

Analysis and Optimum for Air Cooling Thermal Module Using Dimple Vortex Generators

This study examines the airside performance of heat sinks having fin patterns of plate fin (Type I), interrupted fin geometry (Type II), dense vortex generator (Type III), and loose vortex generator (Type IV). Test results indicate that the heat transfer performance is strongly related to the developing and fully developed flow characteristics. The augmentations via vortex generator are relatively effective when the flow is in the developing region whereas they become quite less effective in the fully developed region. This is especially pronounced when operated at a lower frontal velocity. Actually the plain fin geometry outperforms most of the enhanced fin patterns such as of type II and type III at the fully developed region. This is because a close spacing prevents the formation of vortex, and the presence of interrupted surface may also suffer from the degradation by constriction of conduction path. The results suggest that the vortex generators operated at a higher frontal velocity is more beneficial than that of plain fin geometry. In association with the VG-1 criteria (same pumping power and same heat transfer capacity), the results show that effective reduction of surface area can be achieved when the frontal velocities are at 3∼5 m/s and the fin patterns are triangular, triangular attack, or two-groups dimple. The result from the present experiment suggests that the asymmetric combination such as using loose vortex generator (Type IV) can be quite effective. The triangular attack vortex generator is regarded as the optimum enhancement design for it could reduce 12∼15% surface area at a frontal velocity around 3∼5 m/s. The asymmetric design is still applicable even when the fin pitch is reduced to 1 mm.Copyright

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