Local Heat Transfer Measurements Research Articles

Heat transfer measurements and correlations in the transition region for a circular tube with three different inlet configurations

Local forced and mixed convective heat transfer measurements were made in a horizontal circular straight tube with reentrant, square-edged, and bell-mouth inlets under uniform wall heat flux condition. For the experiments the Reynolds number of the liquid ranged from about 280 to 49,000, the Prandtl number varied from 4 to 158, and the Grashof number ranged from about 1000 to 2.5 × 10 5. From the experiments the heat transfer transition Reynolds number range was determined to be about 2000–8500 for the reentrant inlet, 2400–8800 for the square-edged inlet, and 3800–10,500 for the bell-mouth inlet. A correlation for prediction of the developing and fully developed forced and mixed convection heat transfer coefficients in the transition region for each inlet is recommended. Heat transfer correlations for the laminar and turbulent regions for the three inlets are also proposed. Some of the available heat transfer correlations are compared with our experimental data for different inlets.

An Experimental Study of Heat Transfer and Pressure Drop in a Double-Pipe Condenser with U-Bends. Experiments with a Zeotropic Refrigerant Mixture R-114/R113.

Local heat transfer and pressure drop measurements were made during condensation of a nonazeotropic refrigerant mixture R114/R113 in the annulus of a double-tube coil consisting of four straight sections and three U-bends. The inner tube was a corrugated tube having soldered wire fins. The vapor phase mass transfer coefficient exhibited a sawtooth behavior, with those of the U-bends larger than those of the straight sections. The frictional pressure gradient data agreed well with a previously developed empirical equation for the condensation of pure refrigerants. A prediction method for the condensation heat transfer rate was proposed on the basis of the correlations of the vapor phase mass transfer coefficient and heat transfer coefficient of the condensate film. The heat transfer data were correlated by the present prediction method to a mean absolute deviation of 12.9%.

Experimental investigations of heat transfer enhancement and flow losses in a channel with double rows of longitudinal vortex generators

Flow structure, heat transfer, and drag by longitudinal vortices generated by double rows of delta winglets in transition channel flow are investigated for the reduction of the gas side heat transfer resistance of compact heat exchangers. The experiments consist of flow visualization by laser light sheets, liquid crystal thermography for local heat transfer and balance measurements for drag. Angle of attack and channel Reynolds number have been varied. Aligned delta winglet double rows show higher heat transfer enhancement than staggered. The critical angle of attack for the formation of longitudinal vortices is smaller behind the second row than behind the first. Heat transfer enhancements of 80% and drag increases of 160% have been found on wall areas 40 times the winglet area. The ratio of heat transfer enhancement and drag increase is larger for higher Reynolds numbers.

HEAT TRANSFER MEASUREMENTS ON A RIBBED SURFACE AT CONSTANT HEAT FLUX USING INFRARED THERMOGRAPHY

Abstract Convective heat transfer over ribbed surfaces is a very complex process that requires detailed measurements for further understanding. The experimental technique presented in this article is a viable alternative to existing methods for providing local heat transfer measurements on a surface of complex geometry. Infrared (IR) thermography was used in a subsonic wind tunnel to obtain measurements of radiant power over a constant-heat-flux ribbed surface. The results using this procedure show the development of the thermal field near the origin of the ribbed surface. Velocity and pressure drop data illustrate the significant changes in the flow field (near the first ribs), which correlate with the infrared data for the thermal field.

Developing Heat Transfer and Friction in a Ribbed Rectangular Duct With Flow Separation at Inlet

The local heat transfer and pressure drop characteristics of developing turbulent flows in a rectangular duct with an abrupt-contraction entrance and repeated square-rib pairs on the two opposite walls have been investigated experimentally. Both entrance-region and periodic-fully-developed-region results were obtained. Laser holographic interferometry was employed in the local and average heat transfer measurements. The Reynolds number was varied from 5.0 × 103 to 5.0 × 104; the rib pitch-to-height ratios were 10, 15, and 20; and the rib height-to-duct height ratio was kept at a value of 0.13. The results allowed the entry length to be determined and the regions susceptible to hot spots to be located. Semi-empirical heat transfer and friction correlations for the periodic fully developed region were developed. Moreover, performance comparisons between the ribbed and smooth ducts were made under two types of constraint, namely equal mass flow rate and equal pumping power. Finally, the effect of thermal entry length on the length mean Nusselt number was also investigated. The results showed that the length mean Nusselt number ratio was a function of only the duct length and independent of PR and Re, and could be further correlated by an equation of the form Num/Nup = 1 + 1.844/(X/De).

Flow structure and heat transfer in a channel with multiple longitudinal vortex generators

Experimental results of flow structure and heat transfer enhancement by longitudinal vortices in plane channels built by parallel plates in the transition flow regime are presented. The longitudinal vortices are generated by single and double rows of delta half-wing vortex generators punched out of the channel wall. The investigations are performed by a laser light sheet technique for flow visualization and unsteady liquid-crystal thermography for local heat transfer measurements. For an aligned arrangement of two rows of vortex generators, the experiments show that the flow structure in the wake of the second row is qualitatively similar to that of the first row. The peak value of the span-averaged Nusselt number at the wake of the second row is strongly dependent on the spacing of the two rows. For steamwise distances between the two rows of about seven channel heights this peak value is larger than the peak value of the span-averaged Nusselt number at the wake of the first row.

Computer-based areal surface temperature and local heat transfer measurements with thermochromic liquid crystals (TLC)

The experimental technique presented is designed to obtain detailed local heat transfer data on both stationary as well as rotating disc-cavity surfaces applicable to gas turbines. The method employed utilizes thin coatings of thermochromic liquid crystals (TLC) as surface temperature indicators under aerodynamically steady but thermally transient experimental conditions. The color display of the liquid crystals is monitored by a video camera. The video signals are captured in real time by a computer-based color recognition system to extract areal temperature and heat transfer information. Some typical results are presented and compared with-literature data to illustrate the potential of the system.

Condensation of a Nonazeotropic Refrigerant Mixture R114/R113 in Horizontal Annuli With an Enhanced Inner Tube

Local heat transfer and pressure drop measurements are made during condensation of a nonazeotropic refrigerant mixture R114/R113 in the annuli of horizontal double-tube condensers. The inner tube is a 19.1-mm-o.d. corrugated tube with copper wire fins soldered on the outer surface. The outer tubes are smooth tubes with different i.d. of 29.9 and 25.0 mm. The heat transfer coefficient based on the bulk-vapor to wall temperature difference is considerably smaller for R114/R113 mixtures than for pure R113. An empirical equation for the vapor phase mass transfer coefficient is derived, in which the dimensionless parameters are introduced on the basis of the previous results for turbulent single phase flow in smooth and rough tubes with and without surface suction. The measured condensation heat transfer coefficients for R114/R113 mixtures are correlated by the present model to a mean absolute deviation of 14.3 percent.

Incompressible steady aerodynamics using a standard finite element code

Data were also acquired at a - 4-deg angle of attack, which allowed more complete measurement of pressure side heat transfer values. Comparing the -4-deg (pressure side) data with the + 4-deg (suction side) data showed that for the roughened airfoil the pressure side experienced lower heat transfer than the suction side. In computer codes, heat transfer from airfoils is often estimated by using cylinder-in-crossflow heat transfer values in the leading edge region and flat plate heat transfer values farther aft. Figure 6 shows the IRT data for the dense 2, 0-deg, without spray condition compared with the cylinder and flat plate heat transfer values. The heat transfer in the stagnation region for the dense 2 roughened airfoil agrees fairly well with Frossling's5 smooth cylinder laminar flow solution. Moving downstream on the airfoil, the heat transfer drastically increases, reaching a maximum level near s/c of 0.035, and then decreases to a level fairly consistent with turbulent flow flat plate heat transfer values.6 The measured Frossling numbers at specific Reynolds numbers are somewhat higher than their respective flat pjate turbulent values. However, the higher measured heat transfer may be due to the increase in surface area caused by the roughness elements (3-7% increase on each gauge for the dense roughness patterns) that was not taken into account in the data analysis. It may be mentioned here that the maximum heat transfer is in the same general region, if slightly aft, of ice horn growth observed during glaze ice accretion.7 Conclusions Local heat transfer measurements from a roughened NACA 0012 airfoil were successfully obtained in flight and in the NASA Lewis icing research tunnel using the method and apparatus described in this work. Major conclusions resulting from this study are as follows. 1) The addition of roughness to the airfoil surface drastically increased the heat transfer downstream of stagnation. The roughness elements disturbed the laminar boundary-layer flow and in some cases caused a transition to turbulent flow. 2) Comparison of the flight and tunnel roughened surface data showed that the general effect of increased turbulence was a slight increase in heat transfer, especially at the higher Reynolds numbers. 3) Generally, the roughened surface airfoil cases showed the suction side heat transfer monotonically increasing with angle of attack.

Impingement heat transfer under a confined slot jet. Part I: Effect of surface throughflow

AbstractLocal and average heat transfer coefficients were measured for a confined turbulent slot jet impinging on a permeable surface at which there may be throughflow. Local Nusselt number was obtained using a unique porous sensor designed for measurement of local heat transfer at a permeable surface which is subjected to rapidly and widely varying heat transfer. Measurements were performed for a wide range of jet Reynolds number and throughflow rates. Convective heat transfer coefficients was found to be enhanced by throughflow, and the enhancement factor in terms of Stanton number to be independent of jet Reynolds number and of extent of heat transfer area.

Condensation of nonazeotropic refrigerant mixture R114/R113 in horizontal annuli with an enhanced inner tube. Experimental results.

Local heat transfer and pressure drop measurements were made during condensation of a nonazeotropic refrigerant mixture R114/R113 in the annuli of horizontal double-tube condensers. The inner tube was a 19.1mm o.d. corrugated copper tube with soldered wire fins on the outer surface. The outer tubes were smooth tubes with inside diameter Di of 29.9 and 25.0mm. The pressure drop and the heat transfer coefficient based on the bulk vapor-to-wall temperature difference were considerably smaller for R114/R113 than for R113. The vapor phase mass transfer coefficient βn was higher for larger test fluid mass velocity G and Di. At constant values of G and Di, βn was higher for larger condensation mass flux. The radial distribution of the vapor temperature was affected by the vapor velocity, becoming flatter at the outer part of the annulus and steeper near the liquid-vapor interface as the vapor velocity increased.

Heat transfer from smooth and rough in-line tube banks at high Reynolds number

Pressure drop and heat transfer from smooth and rough in-line tube banks in cross-flow have been measured in the Reynolds number range 5 × 10 4 $ ̌ Re $ ̌ 6 × 10 6 . The coolant is pressurized air up to 40 bar. The roughness parameter is varied from k/ d = 0 to 9 × 10 −3 The results from the local heat transfer measurements contribute to the understanding of the complicated flow around the tubes. Also for in-line arrangements a critical Reynolds number exists beyond which the heat transfer is improved while the pressure drop decreases. Finally, the entrance effect on heat transfer is considered.

Local forced convective heat transfer from protruding and flush-mounted two-dimensional discrete heat sources

Experiments are undertaken to measure the local forced convective heat transfer characteristics of an array of two-dimensional discrete heat sources. Air flow rates yielding effective channel Reynolds numbers in the range from 1000 to 10000 are employed. Flush-mounted and protruding heater configurations are investigated in the study. Two different channel wall spacings independent of the heat source protrusion are also studied. Local temperature measurements are made along the uniformly heated surface parallel to the flow direction. The results of the experiments show that protruding heat sources yield higher heat transfer than flush-mounted heat sources at the same channel Reynolds number. The interruption of the thermal boundary layer in the adiabatic sections between the heaters results in heat transfer enhancement. Heat source protrusion yields significantly higher heat transfer rates for the second and subsequent heater for channal Reynolds numbers above 2000. This is explained in terms of a separated flow and vena contracta effect, with eventual downstream transition to turbulent flow, and is borne out by flow visualization experiments. The local heat transfer measurements reveal significant variation in wall temperature across the heater faces, which is strongly affected by the flow structure. Average heat transfer coefficients calculated by spatially integrating the local measurements are correlated and presented in terms of Reynolds number and the dimensionless geometric parameters studied.

An Instrument for Local Radiative Heat Transfer Measurement Around a Horizontal Tube Immersed in a Fluidized Bed

An instrument for the measurement of the radiative component of total heat transfer in a high-temperature gas fluidized bed is described. The main objective of this paper is to emphasize the design, instrumentation, and calibration of this device. The results are presented and discussed elsewhere (Alavizadeh, 1985; Alavizadeh et al., 1985). The design makes use of a silicon window to transmit the radiative heat flux to a thermopile-type heat flow detector located at the base of a cavity. The window material thermal conductivity is sufficiently large to prevent conduction errors due to the convective component of total heat transfer. Also, its transmission and mechanical hardness are well suited for the fluid bed environment. The device has been calibrated using a blackbody source both before and after exposure to a fluidized bed, indicating the effect of the abrasive bed environment on performance. The instrument has been used to measure local radiative heat transfer around a horizontal tube. Typical results for a particle size of 2.14 mm and a bed temperature of 1050 K are presented and discussed to illustrate instrument performance.

Condensation of refrigerants Rll and R113 in the annulus of a double-tube coil with an enhanced inner tube.

Local heat transfer and pressure drop measurements were made during condensation of refrigerants R11 and R113 in the annulus of a double-tube coil consisting of four straight sections and three U-bends. The inner tube was a corrugated copper tube with soldered wire fins. The local heat transfer coefficient in the U-bend was higher than that in the straight section by a maximum of 35%, whereas the pressure drop in the former was about twice as large as that in the latter. The results were compared with the previous work for straight double tubes with the same inner tube as the present ones. Empirical equations for the local heat transfer coefficient and the local frictional pressure gradient were developed for both the U-bend and straight sections.

The Effect of Variations in Stream-Wise Spacing and Length on Convection From Surface Mounted Rectangular Components

The effect of variations in stream-wise spacing and component length on convection from rectangular, surface mounted components in a channel flow are reported. Component dimensions are the same order of magnitude as the channel wall-to-wall spacing. The channel Reynolds number, with air as the coolant, ranged from 670 to 3000. Flow visualization showed that under the above conditions the channel flow is transitional. The effect of variations in component stream-wise spacing on the level of turbulence in the channel and on the interaction between the core of the channel flow and the recirculating flow in cavities between components is discussed. Pressure drop measurements show that the dominant loss mechanism is due to form drag caused by the components. Local heat transfer measurements are made using an interferometer. Analysis of the results shows that the overall heat transfer is properly correlated in terms of a flow Reynolds number based on the component length. At small component Reynolds number, the overall conductance tends towards the laminar smooth wall value. An overall correlation is proposed which includes the effect of component Reynolds number, channel wall-to-wall spacing, and component stream-wise spacing.

Effect of variations in Streamwise Spacing and Length on convection from Surface-Mounted Rectangular Components

The effect of variations in streamwise spacing and component length on convection from rectangular, surface-mounted components in a channel flow are reported. Component dimensions are the same order of magnitude as the channel wall-to-wall spacing. The channel Reynolds number, with air as the coolant, ranged from 670 to 3000. Flow visualization showed that under the above conditions the channel flow is transitional. The effect of variations in component streamwise spacing on the level of turbulence in the channel and on the interaction between the core of the channel flow and the recirculating flow in cavities between components is discussed. Pressure drop measurements show that the dominant loss mechanism is due to form drag caused by the components. Local heat transfer measurements are made using an interferometer. Analysis of the results shows that the overall heat transfer is properly correlated in terms of a flow Reynolds number based on the component length. At small-component Reynolds number, the o...

Local Heat Transfer Measurements in Turbulent Flow Around a 180-deg Pipe Bend

The paper reports extensive connective heat transfer data for turbulent flow of air around a U-bend with a ratio of bend radius:pipe diameter of 3.375:1. Experiments cover Reynolds numbers from 2 × 104 to 1.1 × 105. Measurements of local heat transfer coefficient are made at six stations and at five circumferential positions at each station. At Re = 6 × 104 a detailed mapping of the temperature field within the air is made at the same stations. The experiment duplicates the flow configuration for which Azzola and Humphrey [3] have recently reported laser-Doppler measurements of the mean and turbulent velocity field. The measurements show a strong augmentation of heat transfer coefficient on the outside of the bend and relatively low levels on the inside associated with the combined effects of secondary flow and the amplification/suppression of turbulent mixing by streamline curvature. The peak level of Nu occurs halfway around the bend at which position the heat transfer coefficient on the outside is about three times that on the inside. Another feature of interest is that a strongly nonuniform Nu persists six diameters downstream of the bend even though secondary flow and streamline curvature are negligible there. At the entry to the bend there are signs of partial laminarization on the inside of the bend, an effect that is more pronounced at lower Reynolds numbers.

Use of Melting Point Surface Coatings for Local Convection Heat Transfer Measurements in Rectangular Channel Flows With 90-deg Turns

Centrifugal forces tend to produce secondary flows around and within channel turns that can have significant effects on the convection heat transfer distribution over the channel walls. Local Nusselt number distributions are presented for 90-deg turns of radius ratios 2 and 3 in a 3:1 aspect ratio rectangular cross section duct oriented with the principal walls in the direction of centrifugal force. A method employing the melting phase change of a surface coating was used to determine the local heat transfer rates, and a data analysis procedure needed for quantitative application of the method to internal flows is described.

Local Heat Transfer Measurements Using an Electrically Heated Thin Gold-Coated Plastic Sheet

This paper describes an experimental technique for measuring the local heat transfer coefficient in complex flows. The technique employs a gold-coated plastic sheet to generate a nearly uniform wall heat flux. The sheet is mounted on an insulated substrate of the desired shape and heating is achieved by passing an electrical current through the gold coating. Three applications of this technique are described: heat transfer downstream of an abrupt expansion, heat transfer from a cylinder in crossflow, and nonuniform circumferential heating in a pipe. For each of these applications the effect of the wall conduction on the surface heat flux is evaluated and found to be small. The effect of nonuniformities in the gold coating, and of the resistance temperature coefficient, are also evaluated and likewise are found to be quite small. Methods of correcting for each of these small effects are presented for the above applications.