An investigation of local heat transfer characteristics in a ventilated disc brake with helically fluted surfaces

Local Nusselt numbers in the cooling flow passage of the automobile disc brake with helically fluted surfaces are presented. The flat surface in the flow passage is modified to the helically fluted surface for the purpose of enhancing the heat transfer rate, thereby reducing the thermal stress and deformation in the disc brake. Thermochromic liquid crystals and shroud-transient technique are used to measure spatially-resolved surface temperature distributions, which are used to deduce local Nusselt numbers. The Reynolds number Re ranges from 30,000 to 70,000, the helix angle θ is fixed at 45° and the dimensionless streamwise distance z/d ranges from 1.5 to 4.5. The results show that in general, local Nusselt numbers monotonically decrease with a distance away from both windward and leeward crests of the helical flute and reach a minimum value near its valley for all Re’s and z/d’s tested. The local Nusselt numbers on the helically fluted grooves are maximum 51.6 to 93.7% higher than values measured on the flat surface. The heat transfer enhancement magnitudes become more pronounced with smaller Re and z/d. The largest enhancement occurs at the windward side of the helical flute at z/d=1.5 and Re=30,000. It is also found that at Re=30,000 the average Nusselt numbers on the helically fluted surface are maximum 37% higher than those on the flat surface. The numerical results show that with 10 braking cycles, the temperatures with helically fluted surface are maximum 44.3%, 36.8%, and 36.6% lower than those with the flat surface in the inlet, the center, and the outlet, respectively.     

Condensation heat transfer coefficients of HFC245fa on a horizontal plain tube

Condensation heat transfer coefficients (HTCs) of HCFC22, HCFC123, HFC134a and HFC245fa are measured on a horizontal plain tube 19.0 mm outside diameter. All data are taken at the vapor temperature of 39°C with a wall subcooling temperature of 3–8°C. Test results show the HTCs of newly developed alternative low vapor pressure refrigerant, HFC245fa, on a smooth tube are 9.5% higher than those of HCFC123, while they are 3.3% and 5.6% lower than those of HFC134a and HCFC22, respectively. Nusselt’s prediction equation for a smooth tube underpredicts the measured data by 13.7% for all refrigerants, while a modified equation yielded 5.9% deviation against all measured data. From the view point of environmental safety and condensation heat transfer, HFC245fa is a long-term good candidate to replace HCFC123 used in centrifugal chillers.     

Condensation heat transfer correlation for smooth tubes in annular flow regime

Condensation heat transfer coefficients in a 7.92 mm inside diameter copper smooth tube were obtained experimentally for R22, R134a, and R410A. Working conditions were in the range of 30–40°C condensation temperature, 95–410 kg/m²s mass flux, and 0.15–0.85 vapor quality. The experimental data were compared with the eight existing correlations for an annular flow regime. Based on the heat-momentum analogy, a condensation heat transfer coefficients correlation for the annular flow regime was developed. The Breber et al. flow regime map was used to discern flow pattern and the Muller-Steinhagen & Heck pressure drop correlation was used for the term of the proposed correlation. The proposed correlation provided the best predicted performance compared to the eight existing correlations and its root mean square deviation was less than 8.7%.     

Condensation heat transfer coefficients of HCFC22, R410A,R407C and HFC134a at various temperatures on a plain horizontal tube

In this study, external condensation heat transfer coefficients (HTCs) of HCFC22, R410A, R407C, and HFC134a were measured on a smooth horizontal tube at 30, 39, and 50°C with the wall subcooling of 3–8°C. The results showed that condensation HTCs decreased for all fluids tested with an increase in temperature. This is due mainly to such properties as the saturated liquid density and liquid thermal conductivity. These properties decrease as the temperature increase and accordingly HTCs decrease. The condensation HTCs of R410A are 9.2–19.7% higher than those of HCFC22 while those of R134a are 2.5–10.2% lower than those of HCFC22. Condensation HTCs of R407C, non-azeotropic mixture, are 29.4–34.3% lower than those of HCFC22. Overall, the HTCs of R407C are much lower than those of HCFC22, HFC134a and R410A due to the mass transfer resistance in a diffusion vapor film. Condensation HTCs of HCFC22 and HFC134a are higher than those calculated by Nusselt’s equation by 7.7–11.8% and 4.0–11.1% respectively. On the other hand, HTCs of R407C measured on plain tube, however, are not well predicted by these well-known prediction correlations due to the introduction of mass transfer resistance associated with non-azeotropic mixtures.     

Forced convection heat transfer of steam in a square ribbed channel

An experimental study of heat transfer characteristics of steam in a square channel (simulating a gas turbine blade cooling passage) with two opposite surfaces roughened by 60 deg parallel ribs was performed. The ranges of key governing parameters were: Reynolds numbers (Re) based on the channel hydraulic diameter (30000–140000), entry gauge pressure (0.2Mpa–0.5Mpa), heat flux of heat transfer surface area (5kWm⁻²–20kWm⁻²), and steam superheat (13°C–51°C). The test channel length was 1000mm, while the rib spacing (p/e) was 10, and the ratio of rib height (e) to hydraulic diameter (D) was 0.048. The test channel was heated by passing current through stainless steel walls instrumented with thermocouples. The local heat transfer coefficients on the ribbed wall from the channel entrance to the fully developed regions were measured. The semi-empirical correlation was fitted out by using the average Nusselt numbers in the fully developed region to cover the range of Reynolds number. The correlation can be used in the design of new generation of gas turbine blade cooled by steam.     

Enhancement of pool boiling heat transfer coefficients using carbon nanotubes

In this study, the effect of carbon nanotubes (CNTs) on nucleate boiling heat transfer is investigated. Three refrigerants of R22, R123, R134a, and water were used as working fluids and 1.0 vol.% of CNTs was added to the working fluids to examine the effect of CNTs. Experimental apparatus was composed of a stainless steel vessel and a plain horizontal tube heated by a cartridge heater. All data were obtained at the pool temperature of 7°C for all refrigerants and 100°C for water in the heat flux range of 10–80 kW/m². Test results showed that CNTs increase nucleate boiling heat transfer coefficients for all fluids. Especially, large enhancement was observed at low heat fluxes of less than 30 kW/m². With increasing heat flux, however, the enhancement was suppressed due to vigorous bubble generation. Fouling on the heat transfer surface was not observed during the course of this study. Optimum quantity and type of CNTs and their dispersion should be examined for their commercial application to enhance nucleate boiling heat transfer in many applications.     

Natural convection heat transfer estimation from a longitudinally finned vertical pipe using CFD

In this study, CFD analysis of air-heating vaporizers was conducted. A longitudinally finned vertical pipe was used to represent the air-heating vaporizer in the CFD model. Nitrogen gas was used as the working fluid inside the vertical pipe, and it was made to flow upward. Ambient air, which was the heat source, was assumed to contain no water vapor. To validate the CFD results, the convective heat transfer coefficients inside the pipe, h_i-c, derived from the CFD results were first compared with the heat transfer coefficients inside the pipe, h_i-p, which were derived from the Perkins correlation. Second, the convection heat transfer coefficients outside the pipe, h_o-c, derived from the CFD results were compared with the convection heat transfer coefficients, h_o-a, which were derived from an analytical solution of the energy equation. Third, the CFD results of both the ambient-air flow pattern and temperature were observed to determine whether they were their reasonability. It was found that all validations showed good results. Subsequently, the heat transfer coefficients for natural convection outside the pipe, h_o-c, were used to determine the Nusselt number outside the pipe, Nu_o.. This was then correlated with the Rayleigh number, R_a. The results show that R_a and Nu_o have a proportional relationship in the range of 2.7414×10¹² ≤ Ra ≤ 2.8263×10¹³. Based on this result, a relation for the Nusselt number outside the pipe, Nu_o, was proposed. This paper was recommended for publication in revised form by Associate Editor Man Yeong Ha Hyomin Jeong is currently a professor of Mechanical and Precision Engineering at Gyeongsang Nation University. He received his ph.D. in mechanical engineering from the University of Tokyo in 1992 and he joined Arizona State University as a visiting professor from 2008 to 2009. His research interests are in fluid engineering, CFD, cryogenic system, cascade refrigeration system and ejector system, mechanical vapor compression Hanshik Chung is a professor of Mechanical and Precision Engineering at Gyeongsang National University. He obtianed his Ph.D. in Mechanical Engineering from Donga University. He joined Changwon Master’s College and Tongyeong Fisher National College as an assistant Professor in 1988 and 1993, respectively. His research fields extend into the thermal engineering, heat transfer, solar heating & cooling system, LNG vaporizer optimum, solar cell, hydrogen compressor for fuel cell and making fresh water system from sea water     

Theoretical modeling and numerical solution of stratified condensation in inclined tubes

The heat transfer phenomenon occurring during stratified condensation inside an inclined tube is investigated theoretically and numerically. Differential equations governing the kinematic, dynamic, and thermal aspects for vapor condensation inside inclined tubes, which are derived from a thin film flow modeling, are solved simultaneously. These solutions are achieved by applying an explicit finite difference numerical method to predict the condensation heat transfer coefficient variations along the tangential and axial coordinates. The inclination angle is found to have a significant effect on condensation heat transfer coefficient inside inclined tubes. In addition, in accordance with the given physical and thermal condition of working fluids, there is a specific optimum inclination angle. In this study, the 30°–50° range from the horizontal position is found to be the range of the optimum inclination angle for achieving the maximum condensation heat transfer coefficient, with R134a, R141b, and R11 as the working fluids. The results of the present study are compared with experimental data, and a good agreement is observed between them.     

Performance of heat transfer and pressure drop in a spirally indented tube

In an effort to develop a heat transfer enhancement technique for low temperature applications such as utilization of LNG cold energy, an experiment was carried out to evaluate the heat transfer and the pressure drop performance for a spirally indented tube using ethylene-glycol and water solutions and pure water under horizontal single-phase conditions. The test tube diameter was 14.86 mm and the tube length was 5.38 m. Heat transfer coefficients and friction factors for both inner and outer surfaces of the test tube were calculated from measurements of temperatures, flowrates and pressure drops. Correlations of heat transfer coefficients in the spirally indented tube, which were applicable for laminar and turbulent regimes were proposed for inner, and outer surfaces. The correlations showed that heat transfer coefficients for the spirally indented tube were much higher than those for smooth tubes, increased by more than 8 times depending upon the Reynolds number. The correlations were compared with other correlations for various types of surface roughness. The effect of the Prandtl number on the heat transfer characteristics was discussed. The critical Reynolds number from the laminar flow to the turbulent flow inside the spirally indented tube was found to be around Re=1,000.     

Analysis of a crack approaching a circular hole in cross-ply laminates under biaxial loading

The problem of a crack approaching a circular hole in cross-ply laminates under uniaxial and biaxial loading is investigated in this paper. The effects of material orthotropy, geometry [R/d and a/d], and loading conditions on crack tip singularity are investigated. The stress intensity factors are obtained by the modified mapping collocation method. The present results for an isotropic infinite plate show good agreement with existing solutions. The results for cross-ply laminates show that the stress intensity factors strongly depend on material orthotropy, geometry, and loading condition. The stress intensity factors for cross-ply laminates exist between those for θ=0° and those for θ=90° in the whole range of crack length and decrease as the percentage of 0° plies increases. In the range of small crack length the stress intensity factors for biaxial tension are higher than those for uniaxial tension. In the range of large crack length the stress intensity factors for uniaxial tension are higher than those for biaxial tension.     

Effects of upper inflow area on pool boiling in vertical annulus with closed bottoms

The upper inflow area was changed to identify its effects on the pool boiling heat transfer of saturated water at atmospheric pressure in a vertical annulus with closed bottoms. The inside surface of a 25.4 mm diameter tube was heated. The ratio between the gaps measured at the upper and lower regions of the annulus ranged from 0.18 to 1. Two different lengths of modified gap were investigated. The effects of the inflow area on heat transfer became evident as the heat flux increased and the gap ratio decreased. If the gap ratio was smaller than 0.51 and the height of the interrupter was 10 mm, a significant change in heat transfer was observed. This was attributed primarily to the formation of a lumped bubble around the upper regions of the annulus and the generation of active liquid agitation in the annular gap space.     

A three-crucible version of thermal analysis

The operating technique and benefits of three-crucible differential thermal analysis are discussed. Using this method, the temperatures and heat effects of iron phase transitions were determined: 773±3°C (α→β), 923±4°C and 10.2±0.8 J/g (β→γ), 1400±3°C and 17.0±1.0 J/g (γ→δ), and 1539±4°C and 250±20 J/g (°→liquid). The temperature of (α→β) beryllium transition (1274±5°C), its melting point (1292±5°C), and the interval of BCC Be (18±5°C) were also measured. The total heat of both trnasitions is 15.1±0.7 kJ/mol, composed of the heat of the α→β transition (7.5±0.4 kJ/mol) and the heat of melting (7.6±0.4 kJ/mol). *** DIRECT SUPPORT *** A31DG005 00013     

Influences of the wavy surface inserted in the middle of a circular tube heat exchanger on thermal performance

Numerical investigations on flow topology, heat transfer behavior and performance evaluation in a circular tube inserted with various configurations of wavy surfaces, Inclined wavy surface (IWS), V-downstream wavy surface (VDWS), V-Upstream wavy surface (VUWS) are presented. The effects of the flow attack angles; 20°, 30°, 45° and 60° are studied for the Reynolds numbers, Re = 100-2000. The numerical results are compared with the smooth circular tube with no wavy surface and the previous works. It is found that the IWS, VDWS and VUWS can produce longitudinal vortex flow and impinging jet of the fluid flow like inclined baffle, V-downstream baffle and V-Upstream baffle, respectively, but give lower friction loss. The flow phenomena created by the wavy surfaces help to augment the heat transfer rate and thermal performance in the test tube. In the range studied, the order of enhancement for heat transfer rate is around 1.40-3.75, 1.60-6.25 and 1.30-5.80 times higher than the smooth tube for IWS, VDWS and VUWS, respectively. Moreover, the maximum thermal performance, presented in terms of the Thermal enhancement factor (TEF), is found to be about 1.60, 2.40 and 2.10, respectively, for IWS, VUWS and VDWS.     

An analytic study on laminar film condensation along the interior surface of a cave-shaped cavity of a flat plate heat pipe

An analytic approach has been employed to study condensate film thickness distribution inside cave-shaped cavity of a flat plate heat pipe. The results indicate that the condensate film thickness largely depends on mass flow rate and local velocity of condensate. The increasing rate of condensate film for circular region reveals about 50% higher value than that of vertical region. The physical properties of working fluid affect significantly the condensate film thickness, such as the condensate film thickness for the case of FC-40 are 5 times larger than that of water. In comparison with condensation on a vertical wall, the average heat transfer coefficient in the cave-shaped cavity presented 10-15% lower values due to the fact that the average film thickness formed inside the cave-shaped cavity was larger than that of the vertical wall with an equivalent flow length. A correlation formula which is based on the condensate film analysis for the cave-shaped cavity to predict average heat transfer coefficient is presented. Also, the critical minimum fill charge ratio of working fluid based on condensate film analysis has been predicted, and the minimum fill charge ratios for FC-40 and water are about Ψ_crit= 3-7%, Ψ_crit=0.5-1.3%, respectively, in the range of heat fluxq” = 5-90kW/²     

Measurement of developing turbulent flow in a U-bend of circular cross-section

Hot-wire measurements of the full mapping of the velocity and Reynolds stress components are reported for developing turbulent flow in a strongly curved 180 deg pipe and its tangents. A slanted wire is rotated into 6 orientations and the voltage outputs from wires are combined to obtain the mean velocity and Reynolds stress components. The strength of secondary flow reaches up to the 28% of bulk mean velocity. The strong counter-rotating vortex pair induced by the transverse pressure gradient and centrifugal force imbalance grows up to Θ = 67.5° into the bend. But the vortex pair breaks down into two cell pattern after Θ=90° Core vortex formation and reversal of secondary flow direction along the bend symmetry plane is cleanly found in the secondary vector plot. At Θ=67.5° and Θ = 90° into bend a large “trough” develops in the longitudinal velocity toward the inside of the bend due to the breakdown of secondary flow. In the bend, the mean longitudinal velocity component changes little after Θ=90°, but secondary flow never achieves fully-developed state. Similar behaviors are observed in the radial and circumferential stresses.     

A study on the improvement of heat transfer performance in low temperature closed Thermosyphon

The study focuses on the heat transfer performance of two-phase closed thermosyphons with plain copper tube and tubes having 50, 60, 70, 80, 90 internal grooves. Three different working fluids (distilled water, methanol, ethanol) are used with various volumetric liquid fill charge ratio from 10 to 40%. Additional experimental parameters such as operating temperature and inclination angle of zero to 90 degrees are used for the comparison of heat transfer performance of the thermosyphon. Condensation and boiling heat transfer coefficients, heat flux are obtained using experimental data for each case of specific parameter. The experimental results are assessed and compared with existing correlations. The results show that working fluids, liquid fill charge ratio, number of grooves and inclination angle are very important factors for the operation of thermosyphons. The relatively high rate of heat transfer is achieved when the thermosyphon with internal grooves is used compared to that with plain tube. The optimum liquid fill charge ratio for the best heat transfer performance lies between 25% and 30%. The range of the optimum inclination angle for this study is 20°-30° from the horizontal position.     

Thermal characteristics of a multichip module using PF-5060 and water

The experiments were performed by using PF-5060 and water to investigate the thermal characteristics from an in-line 6 x 1 array of discrete heat sources for simulating the multichip module which were flush mounted on the top wall of a horizontal, rectangular channel of aspect ratio 0.2. The inlet temperature was 15°C for all experiments, and the parameters were the heat flux of simulated VLSI chips with 10, 20, 30, and 40W/cm² and the Reynolds numbers ranging from 3,000 to 20,000. The measured friction factors for PF-5060 and water gave a good agreement with the values predicted by the modified Blasius equation within ±6%. The chip surface temperatures for water were lower by 14.4-21.5°C than those for PF-5060 at the heat flux of 30W/cm². From the boiling curve of PF-5060, the temperature overshoot at the first heater was 3.5°C and was 2.6°C at the sixth heater. The local heat transfer coefficients for water were larger by 5.5-11.2% than those for PF-5060 at the heat flux of 30W/cm², and the local heat transfer coefficients for PF-5060 and water reached a uniform value after the fourth row. This meant that the thermally fully developed condition was reached after the fourth row. The local Nusselt number data gave the best agreement with the values predicted by the Malina and Sparrow’s correlation and the empirical correlations for Nusselt number were provided at the first, fourth and sixth rows for a channel Reynolds number over 3,000.     

An experimental study on the thermal performance of a concentric annular heat pipe

Concentric annular heat pipes (CAHP) were fabricated and tested to investigate their thermal characteristics. The CAHPs were 25.4 mm in outer diameter and 200 mm in length. The inner surface of the heat pipes was covered with screen mesh wicks and they were connected by four bridge wicks to provide liquid return path. Three different heat pipes were fabricated to observe the effect of change in diameter ratios between 2.31 and 4.23 while using the same outer tube dimensions. The major concern of this study was the transient response as well as isothermal characteristics of the heat pipe outer surface, considering the application as uniform heating device. A better performance was achieved as the diameter ratio increased. For the thermal load of 180 W, the maximum temperature difference on the outer surface in the axial direction of CAHP was 2.3°C while that of the copper block of the same outer dimension was 5.9°C. The minimum thermal resistance of the CAHP was measured to be 0.04°C/W. In regard to the transient response during start-up, the heat pipe showed almost no time lag to the heat source, while the copper block of the same outer dimensions exhibited about 25 min time lag.     

Two-phase flow heat transfer of R-134a in microtubes

The characteristics of the two-phase flow heat transfer of R-134a in microtubes with inner diameters of 430 μm and 792 μm were experimentally investigated. The effect of the heat flux on the heat transfer coefficient for microtubes was significant before the transition quality. The boiling number expressed the interrelation between the heat flux and the mass about the heat transfer coefficients. The smaller microtube had greater heat transfer coefficients; the average heat transfer coefficient for the tube A (D _i = 430 μm) was 47.0% greater than that for the tube B (D _i = 792 μm) at G = 370 kg/m²·s and q″ = 20 kW·m². A new correlation for the evaporative heat transfer coefficients in microtubes was developed by considering the following factors: the laminar flow heat transfer coefficient of liquid-phase flow, the enhancement factor of the convective heat transfer, and the nucleate boiling correction factor. The correlation developed in present study predicted the experimental heat transfer coefficients within an absolute average deviation of 8.4%.     

An experimental study on the pressure drop and heat transfer through straight and curved small diameter tubes

A tube type heat exchanger is often the only solution when minimum pressure loss is a requirement. In addition, small diameter tubes are preferable because of an increased heat transfer area within an acceptable pressure loss limit. The present work reports on both an analytic model and experimental results with regards to the pressure drop and heat transfer characteristics of compact straight, C-curved, and U-curved tubes. The inner diameter of the tube (D) for our selected heat exchanger type was 1.26 mm with a thickness of 0.12 mm and a total length of 150.8 D. For the experiment, pressurized nitrogen gas bottles were used rather than an air compressor system in order to simplify the test facility. Hence the pressure conditions were easily set at 10, 30, and 50 bar corresponding to a range of Reynolds numbers from 10000 to 50000. To elevate the air temperature outside the tube (from 100°C to 400°C), an electric furnace was installed around the “test tube”. An analytic model to determine the pressure loss through curved tubes-referred to as the modified friction factor- is proposed. Good agreement was found between the modified friction factor and existing correlations, thus confirming the suitability of this model for determining pressure losses for different shape of tubes. The average measured Nusselt numbers were within 10- 15% of the Dittus-Boelter and Gnielinski correlations.