Shou-Shing Hsieh

Liquid flow in a micro-channel

An experimental study with MPIV (micro particle image velocimetry) flow visualization of incompressible liquid flow in a microchannel is presented. Deionized water served as a working medium. The channel was microfabricated in a PMMA substrate using an excimer laser, and was 115 µm deep, 200 µm wide and 24 000 µm long with a hydraulic diameter of about 146 µm. The pressure drop between the inlet and the outlet of the duct as well as mass flow rates were measured and friction factors were calculated for different Reynolds numbers. Data were discussed and compared with those of previous investigations of similar studies. Moreover, time evolutions of the microflow at the middle of the microchannel at Re = 50, 100, 470 and 900 were photographed. Hydrodynamic entry length correlations were found for both laminar and turbulent flow in the present microchannel.

Passive mixing in micro-channels with geometric variations through µPIV and µLIF measurements

Five passive micro-mixers were designed, fabricated and tested, and were compared to that of the conventional T-shaped channel in order to examine the mixing performance by increasing the lateral convection/or related mixing mechanisms. Electro-osmotic pumping was used to generate fluid velocities in the creeping flow regime (Re 1) in order to achieve the mixing enhancement by chaotic advection. Iodine dye (qualitative) and rhodamine B (quantitative) mixing experiments were visualized and measured through conventional microscopes and with µLIF techniques. µPIV measurements were used for 2D velocity vectors and stretching calculations. Steady-state results show that the alternating wavy-regime (chaotic mixers) micro-channels and inline aligned elements (which alter the flow direction laterally) in micro-channels can induce stirring and improve the mixing significantly even at extremely low Reynolds numbers.

Nucleate pool boiling from coated surfaces in saturated R-134a and R-407c

Pool nucleate boiling heat transfer experiments from coated surfaces immersed in saturated R-134a and R-407c with porous aluminum, copper, molybdenum, and pitted coating on a copper surface were conducted. The influence of coating thickness and porosity on heat transfer and boiling hysteresis were studied. The enhanced surface heat transfer coefficients are 2.5 times higher than those of the smooth surface. Furthermore, the effects of refrigerant thermophysical properties are also presented and discussed. The experimental data for plasma and flame spray coating surfaces were correlated in terms of the Reynolds number based on a mean pore diameter and porosity, Jakob number, constant heat flux number and geometric scale factor to provide a thermal design basis for engineering application.

Nucleate pool boiling characteristics from coated tube bundles in saturated R-134a

Abstract Nucleate pool boiling heat transfer from plasma coated copper tube bundles with porous copper (Cu) immersed in saturated R-134a was experimentally studied. The bundle is composed of 15 tubes (of which the number of heated/instrumented tubes was varied) arranged in four different configurations with a pitch-to-diameter ratio of 1.5. The influences of various parameters, for instance, bundle arrangements and heat flux were clarified. Tests were conducted with both increasing and decreasing the heat flux. The data presented indicated that at low heat fluxes, the vertical-in-line tube bundles have the highest bundle factor. A configuration factor was proposed which can be used to characterize the geometric arrangements of the bundles.

Natural convection of opposing/assisting flows in vertical channels with asymmetrically discrete heated ribs

Abstract Both qualitative flow visualization as well as laser holographic interferometry and quantitative temperature measurement are presented for natural convection of air layers in vertical channels (channels A and B) with asymmetrically discrete heated ribs wherein the opposing flow is in channel A and the assisting flow is in channel B. Based on the analyses of the photographs and interferograms, it is suggested that the turbulent flow should be expected when the local modified Rayleigh number is in the range of 1.29 × 10 7 -9.43 × 10 9 for both channels. The heat transfer data and flow visualization photographs indicate that the present rib geometry and the stratification are two major reasons influencing the temperature of the heated ribs. Average Nusselt number correlations are established in the form for channels A and B, respectively. The power dependence strongly indicates that the flow in channel A is turbulent while the flow channel B is laminar.

Turbulent heat transfer and flow characteristics in a horizontal circular tube with strip-type inserts. Part II. Heat transfer

This paper is the second of two papers that presents the results of an extensive study of turbulent heat transfer and pressure drop in a horizontal tubes with strip-type inserts. Experimental data were taken for air for a class of strip inserts (longitudinal strip and crossed-strip). The insert was characterized by the parameters of 1 6 AR 6 5 and R � ( ¼ 0.5 and 1). Friction factor data (from Part I) and temperature measurements were used to understand the underlying physical phenomena responsible for the heat transfer enhancement for 6500 6 Re 6 19 500. Nusselt numbers were between four and two times the bare tube values at low Re and high Re, respectively. Performance evaluation index R1 (constant pumping power) and R2 (constant heat duty) were made and an optimum condition would be thus found.

Developing three-dimensional laminar mixed convection in a circular tube inserted with longitudinal strips

Abstract A finite-difference numerical study with a boundary-fitted coordinate system was made to investigate the three-dimensional, steady-state, laminar entrance flow of an axially uniformly heated horizontal tube with different adiabatic inserts. Parametric calculations have been performed to determine the nature and effect on thermally driven secondary flows of Reynolds number (100 ⩽ Re ⩽ 1000), Grashof number (103 ⩽ Gr ⩽ 106), aspect ratio (1 ⩽ AR ⩽ 8) and radius ratio (0 ⩽ R∗ ⩽ 1) . Typical streamline patterns, isotherms and axial velocity along the flow direction are obtained. Local friction factor, Nusselt number and entrance length in the hydrodynamically and thermally developing, and fully developed regions are presented and discussed. Furthermore, flow visualization was made and compared with the corresponding numerical results. The agreement is good for Re = 500 at the two different downstream distances.

Laminar forced convection from surface-mounted ribs

Abstract A study for both numerical and experimental aspects on low speed forced convective heat transfer near two-dimensional transverse ribs in which the walls are held at a uniform temperature is presented. The effects of aspect ratio and Reynolds number as well as the initial boundary layer thickness are examined and discussed. The numerical technique is based on a power law combined with a false stream function-vorticity form. It is found that the temperature data agree reasonably well with those taken from the experiments qualitatively. The results are further correlated by an equation of the form Nu av = 0.44(Re s ) 0.465 (d/s) −0.36 (w/s) −0.265 .

Turbulent heat transfer and flow characteristics in a horizontal circular tube with strip-type inserts. Part I. Fluid mechanics

Abstract This paper is the first of two papers that present the result of a study of turbulent flow and pressure drop in a horizontal tube with strip type inserts. Experimental data taken for air for a class of strip type inserts (longitudinal, LS and cross, CS inserts) used as a tube side heat transfer augmentative device for a single-phase cooling mode operation are presented. To broaden the understanding of the underlying physical phenomena responsible for the heat transfer enhancement, flow mechanisms through velocity measurements are combined with pressure drop measurements to develop friction factor correlations for 6500⩽Re⩽19 500 where Re is the Reynolds number. Friction factor increases were typically between 1.1 and 1.5 from low Re(≅6500) to high Re(≅19 500) with respect to bare tubes.

Evaporative heat transfer and enhancement performance of rib-roughened tube annuli with refrigerant 114

Abstract Heat transfer and pressure drop measurements were performed on three rib type roughened tube annuli (4.67–6.93 mm D h , 9–16 ribs, rib pitch 19.7–39.4 mm, rib height 4 mm, rib width 15 mm, rib angle 30–60°) with two-phase flow of refrigerant R-114 under evaporating condition. The data were compared with the performance of a smooth tube annulus ( 6.5 mm D h ). Based on the same heat transfer area (≈4.24 cm 2 ) of the test section, changes in mass velocity range (63.55–129.86 kg m−2 s−1) and heat flux level (2.01–2.95 × 10 4 W m −2 ) on heat transfer coefficient and pressure drop with the varied quality were conducted and the influences were studied. Enhancement performance ratios are also presented and discussed. Moreover, flow boiling experiments with rib type roughened surfaces using R-114 through flow visualization were explored to broaden our fundamental understanding of the boiling heat transfer mechanism.