Begell House Inc.
Journal of Enhanced Heat Transfer
JEH(T)
1065-5131
19
4
2012
ENHANCED PERFORMANCE OF ENVIRONMENTALLY FRIENDLY REFRIGERANT MIXTURE WITH AND WITHOUT MAGNETIC FIELD EFFECT
293-300
10.1615/JEnhHeatTransf.2012003137
Kolandavel
Mani
Coimbatore Institute of Technology; Department of Mechanical Engineering, Refrigeration and Air Conditioning Laboratory, Panimalar Engineering College, Chennai-600123, India
V.
Selladurai
Department of Mechanical Engineering, Refrigeration and Air Conditioning Laboratory, Coimbatore Institute of Technology, Coimbatore-641 014, India
hydrocarbon
refrigerants
ODP
global warming
CFC
magnetic field
eco-friendly
The performance enhancement of hydrocarbon mixture refrigerant R290/R600 (79/21 by wt %) was analyzed as an alternative to R12 and R134a, with and without the effect of magnetic field force at the condenser outlet. Six magnets with a Gauss level of 4000 each were attached on the refrigerant full liquid line of the condenser outlet. Experiments were conducted with R12, R134a, and R290/R600 mixture refrigerant at different condensing and evaporating temperatures. The parameters investigated were refrigeration capacity, compressor power, and coefficient of performance. The test results showed that the R290/R600 refrigerant mixture had a 30.5−78% higher refrigeration capacity and a 4.6−28.2% higher coefficient of performance than that with R12 without magnetic field effect. The refrigerant R134a showed a slightly lower coefficient of performance than that of R12. The magnetic field force lowered the compressor power consumption by 1.5−2.4% and enhanced the coefficient of performance of the system by 1.6−2.5% compared with that with no magnets. The R290/R600 (79/21 by wt %) refrigerant blend can be considered as an alternative refrigerant for R12 and R134a, with or without the effect of magnetic field.
FALLING FILM EVAPORATION OF PURE REFRIGERANT HCFC123 IN A PLATE-FIN HEAT EXCHANGER
301-311
10.1615/JEnhHeatTransf.2012001693
Junichi
Ohara
Department of Ocean Mechanical Engineering, National Fisheries University, Nagatahonmachi 2-7-1, Shimonoseki, Yamaguchi, Japan
Shigeru
Koyama
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan; Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga-koen 6-1, Kasuga-shi, Fukuoka 816-8580, Japan
plate-fin heat exchanger
falling film evaporation
flow pattern
In the present study, the characteristics of heat transfer and flow patterns are investigated experimentally for the falling film evaporation of pure refrigerant HCFC123 in a vertical rectangular channel with a serrated-fin surface. The refrigerant liquid is supplied to the channel through 37 holes of a distributor. The liquid flowing down vertically is heated electrically from the rear wall of the channel and evaporated. To directly observe the flow patterns during the evaporation process, a transparent vinyl chloride resin plate is placed as the front wall. The experimental parameters are as follows: the mass velocity G = 28−70 kg/(m2·s), the heat flux q = 20−50 kW/m2, and the pressure P ≈ 100 kPa. It is clarified that the heat transfer coefficient α depends on G and q in the region of vapor quality x ≥ 0.3 while there is little influence of G and q in the region x ≤ 0.3. From the direct observation using a high-speed video camera and a digital still camera, flow patterns are classified into five typical patterns: plane liquid film, wavy liquid film, liquid film accompanied with a dry patch, liquid film accompanied with dripping, and liquid film accompanied with mist. Then the relation between heat transfer and flow pattern is clarified. The results of heat transfer characteristics are also compared with some previous correlation equations.
MIXED CONVECTIVE FLOW AND HEAT TRANSFER THROUGH A HORIZONTAL CHANNEL WITH SURFACE MOUNTED OBSTACLES
313-329
10.1615/JEnhHeatTransf.2012003035
Arun K.
Saha
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
Twinkle
Malik
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, 208 016, India
horizontal channel
mixed convection
obstacles
fluid flow
Mixed convective flow and heat transfer has been performed in a horizontal channel with built-in obstacles on one of the channel walls. The channel blockage is taken to be 20%. Two-dimensional time-dependent Navier-Stokes and energy equations are solved in a Cartesian coordinate system using higher order discretization. Time integration of the governing equations reveals the critical Reynolds number of unsteadiness to be between 525 and 550. The effect of Richardson number on unsteady flow at a fixed Reynolds number of 600, at which the flow is unsteady laminar, has been investigated. The Richardson number, the parameter of interest of the present study, is varied between −1.0 and 1.0. The fluid chosen for the present study is air whose Prandtl number is 0.7. The flow and heat transfer show good correlation, and intense mixing with complex flow structures has been observed downstream of the obstacle. The heat transfer is found to be higher for the positive buoyancy while no significant variation is observed for negative buoyancy or Richardson number.
COMPENSATION OF THREE-DIMENSIONAL HEAT CONDUCTION INSIDE WALL IN HEAT TRANSFER MEASUREMENT OF DIMPLED SURFACE BY USING TRANSIENT TECHNIQUE
331-341
10.1615/JEnhHeatTransf.2012003016
Satomi
Nishida
Department of Mechanical Systems Engineering, Tokyo University of Agriculture & Technology, Naka-cho 2-24-16, Koganei, Tokyo 184-8588, Japan
Akira
Murata
Department of Bio-Functions and Systems Science, Tokyo University of Agriculture and Technology, Nakacho, Koganei, Tokyo, Japan
Hiroshi
Saito
Mechanical Systems Engineering Course, Tokyo Metropolitan College of Industrial Technology, 1-10-40 Higashi-Ohi, Shinagawa, Tokyo 140-0011,
Japan
Kaoru
Iwamoto
Department of Mechanical Engineering, Tokyo University of Science, Noda-shi, Chiba 278-8510; Department of Mechanical System Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan
heat conduction
numerical simulation
transient technique
dimpled surface
The transient technique using infrared thermography or liquid crystal has been widely used for measuring the distribution of local heat transfer coefficients. In this technique, wall surface temperature is measured, and the heat transfer coefficient is calculated so as to accord the measured temperature with the theoretical solution of a one-dimensional heat conduction problem. In actual cases of complicated surface geometry, however, three-dimensional heat conduction, caused by the three-dimensionality of the wall surface and the distribution of heat transfer coefficient, occurs in the wall. In this study, the heat transfer enhancement on the hemispherically dimpled surface was measured with an infrared camera, while the three-dimensional heat conduction in the wall was numerically calculated. In the compensation process, modification of the heat transfer coefficient was repeated until the numerical result agreed with the measured surface temperature. The present results showed that the heat transfer coefficient near the dimple edge was overrated, while that within the cavity was underrated. The maximum error induced by the three-dimensional heat conduction was +50% on the leading edge of the dimple, and the error in the other area was about −20% at most. At the dimple edge, the convex geometry increased the surface area where the heat flew into the wall, and consequently the temperature rise became larger than the flat part. On the other hand, within the dimple, the concave geometry formed the radially expanding heat conduction area, and the temperature became lower. The principal factor contributing to the error of the measurement is the three-dimensionality of the surface.
THE EFFECTS OF A CROSS-FLOW SYNTHETIC JET ON SINGLE-PHASE MICROCHANNEL HEAT TRANSFER
343-358
10.1615/JEnhHeatTransf.2012003386
Ruixian
Fang
Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA
Wei
Jiang
Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina 29208, USA
Jamil A.
Khan
Department of Mechanical Engineering, Laboratory for Applied Heat Transfer, University of South Carolina, Columbia, SC 29208, USA
synthetic jet
microchannel
heat transfer enhancement
heat sink
electronics cooling
The present study is an experimental investigation of a novel hybrid cooling scheme which combines a microchannel heat sink with a synthetic jet actuator. The heat sink consists of a single rectangular microchannel with dimensions of 550 μm in width, 500 μm in depth, and 26 mm in length. The synthetic jet actuator with a 100-μm-diameter orifice is placed vertically above the microchannel and 5 mm downstream from the channel inlet. The microjet is synthesized from the fluid flowing through the microchannel. Periodic disturbance is generated when the synthetic jet interacts with the channel flow. Heat transfer performance is enhanced as local turbulence is generated and propagated downstream of the channel. The scale and frequency of the disturbance can be controlled by changing the driving voltage or frequency of the piezoelectric driven synthetic jet actuator. The effects of a synthetic jet on microchannel heat transfer performance were evaluated as a function of the microchannel flow rate, the jet operating voltage, and frequency, respectively. Approximately 40%−50% heat transfer enhancement are achieved for some test cases. The pressure drop across the microchannel increases slightly with the synthetic jet. This study concludes that the synthetic jet can effectively enhance single-phase microchannel heat transfer performance.
EFFECT OF DIMPLE ARRANGEMENTS ON THE TURBULENT HEAT TRANSFER IN A DIMPLED CHANNEL
359-367
10.1615/JEnhHeatTransf.2012004509
Y. O.
Lee
Korea Power Engineering Company, Yongin, Korea
Joon
Ahn
School of Mechanical Engineering, Kookmin University, Seoul National University, Seoul, Korea
Jungwoo
Kim
Seoul National University of Science and Technology
Joon Sik
Lee
School of Mechanical and Aerospace Engineering, Seoul National University, San 56-1. Sillim-dong Gwanak-gu. Seoul 151-742 Korea
dimpled channel
large eddy simulation
heat transfer
pressure drop
Large eddy simulations (LES) have been conducted for the flow and heat transfer in a dimpled channel at the bulk Reynolds number of 20,000 to study the effect of dimple arrangements. Dimples are placed on one side of the channel in three canonical arrangements: in-line, staggered, or with cylindrical grooves. Heat transfer is not actively augmented at the spanwise gap between dimples for the in-line arrangement, while the problem can be cured by adopting the staggered arrangement. The cylindrical grooves provide heat transfer enhancement effect as much as the staggered arrayed dimples do.
FORCED CONVECTION HEAT TRANSFER OF A SINGLE V-BENT CIRCULAR FIN-TUBE HEAT EXCHANGER
369-378
10.1615/JEnhHeatTransf.2012002877
Jonghwi
Lee
Kunsan National University
Hie-Chan
Kang
Kunsan National University
heat transfer
forced convection
heat exchanger
fin
pressure drop
The purpose of the present study is to investigate the flow resistances and heat transfer characteristics of V-bent circular fin-tube heat exchangers. Four types of V-bent fin in which the fin areas are identical but the bent portions are different have been tested numerically. The heat transfer, pressure drop, fin temperature, fin efficiency, and shear stress are discussed. With an increase of 37% in the area of the V-bent portion, the heat transfer coefficient increases by up to 32% in the present test range.
THE ENHANCEMENT EFFECTS OF A PLUME OF RISING BUBBLES ON NATURAL CONVECTION FROM A HEATED VERTICAL PLATE
379-395
10.1615/JEnhHeatTransf.2012004080
David
Donoghue
Department of Mechanical & Manufacturing Engineering, Trinity College Dublin, Ireland
Brian
Donnelly
Department of Mechanical & Manufacturing Engineering, Trinity College Dublin, Ireland
Darina B.
Murray
Department of Mechanical and Manufacturing Engineering, The University of Dublin, Trinity College, College Green, Dublin 2, Ireland
bubble
heat transfer
buoyancy driven
bubble plume
Rising bubbles have been found to significantly enhance heat transfer from a heated surface. This is associated with the bubble acting like a bluff body, displacing fluid as it moves, and also via the wake generated by the bubble, which increases fluid mixing. The present research explores the motion of both a single ellipsoidal bubble and a plume of rising bubbles and their influence on heat transfer from a heated vertical surface. Prior to this, the effect on heat transfer of a stream of bubbles rising past a vertical surface has received little attention. By simultaneously measuring the time varying heat transfer via a hot film sensor and the bubble dynamics by means of high speed cameras, an improved understanding of the heat transfer mechanism is achieved. It was found that a bubble mean path normal to the heated surface produced higher initial enhancements in heat transfer when compared to bubble motion in a plane parallel to the surface, although this effect was insignificant when a plume of bubbles was investigated.