Heat Transfer Coefficient Of Tube Research Articles

Numerical Study on Local Flow Field and Temperature Field of Helical Baffles Heat Exchanger

Models of continuous helical baffles and quarter helical baffles heat exchangers are established. With large CFD analysis software FLUENT, the distribution characteristics of local flow field and temperature field of helical baffles heat exchangers are investigated through numerical simulation. And comprehensive performances of both heat exchangers are compared and studied with entransy dissipation. The results show:In the research of heat exchanger, the evaluation standard of entransy dissipation is consistent with that of comprehensive performances. When the flow of shell side is the same, the entransy dissipation value of continuous helical baffles heat exchanger is less than that of quarter helical baffles heat exchanger. The distribution characteristics of local flow field and temperature field of both heat exchangers present the same rules, in the central part of shell side, heat transfer coefficient of tubes is the highest. Along the radial direction, both helical flow and heat transfer coefficient of tubes first increase then decrease. Compared to continuous baffles, the discontinuity of baffles could enhance local heat transfer. Yet, when flow increases to an extent, the local heat transfer in the center will be weakened. The results provide theoretical basis for the improvement of structure forms of helical baffles.

Micro Convective Heat Transfer of Gas Flow Subject to H and T Boundary Conditions

This paper focuses on experimental and numerical analysis of convective heat transfer characteristics of pressure-driven gaseous flows through microtubes, which is frequently encountered in practical application of microfluidic devices accommodating gas flow, heat transfer and/or chemical reactions at microscale. The present work has been carried out with the objectives to: (i) verify the applicability of conventional theory for the prediction of internal forced convection heat transfer coefficient for tubes having an inner diameter lower than 1 mm and (ii) check the performance of some specific correlations proposed for the analysis of forced micro convection with gases in the last decades. Single commercial stainless steel microtubes are tested with inner diameters ranging from 1 mm down to 0.17 mm. The most common thermal boundary conditions, namely uniform heat flux (H boundary condition) and uniform wall temperature (T boundary condition), have been implemented by applying Joule heating on external s...

Experimental and numerical determination of heat transfer coefficient between oil and outer surface of monometallic tubes finned on both sides with twisted internal longitudinal fins

Results of thermal investigations of monometallic tubes finned on both sides used in heat exchangers are presented in this article. Investigated tubes differ in twist and inclination angles of internal fins. In experiments a transform oil was cooled and air was a coolant. Heat transfer coefficient values for the inner side of the tube based on correlation equations from literature were compared with the experimental and numerical ones. The aim of this research is to specify the meaning of correlation equations for determination of heat transfer coefficients of tubes finned on both sides with twisted longitudinal internal fins. The change in heat flux in a tube when the twist angle of the internal fins varies is shown. Results of computations are presented in the form of useful drawings, which can be readily applied in practice.

Effect of non-uniform air velocity distribution on evaporator performance and its improvement on a residential air conditioner

The effect of non-uniform air velocity distribution on performance of a multi-circuit evaporator was studied numerically and experimentally. The non-uniform air velocity distribution on evaporator was measured experimentally, and its corresponding evaporator performance was tested and approximated numerically. The evaporator performance was also investigated numerically at the equal airflow rate but under uniform air distribution. The results showed that the evaporator capacity under non-uniform air distribution was decreased by 7.78% than that under uniform air distribution. The decrease in evaporator capacity was mainly attributed to smaller overall heat transfer coefficient of lower tubes with smaller air velocity in first row because overall tube heat transfer coefficient varied remarkably at smaller air velocity and gently at greater air velocity. Two triangle air guide plates were installed on sheet backing of evaporator to deflect part of airflow towards evaporator bottom to increase air velocity of lower tubes and to decrease the area that smaller air velocity covered. The experimental results showed air guide plates decreased blend loss of high temperature air leaving upper circuits and low temperature air leaving lower circuits, thus decreasing outlet air dry and wet temperatures. Finally, the cooling capacity and EER were increased by 3.02% and 5.1%, respectively. ► Evaporator performance between non-uniform and uniform air distributions are compared. ► Evaporator capacity under non-uniform air distribution decreased by 7.78%. ► Overall heat transfer coefficient was a dominant factor of evaporator capacity decrease. ► Air guide plates on sheet backing of evaporator could improve evaporator performance.

Experimental Study on Condensation of R134a inside Horizontal Inner-Micro-Fin Tubes

An experimental study of condensation heat transfer of R134a on horizontal inner enhanced tubes was conducted. The tested tubes were inner-micro-fin tubes, named tube A and tube B, respectively. The tested pieces were double-pipe condensers. The glycol solution flowed in the space between outer surface of the enhanced tube and inner surface of outer tube. In the experiment, condensing temperature inside the enhanced tube was 51°C, and the flow velocity of glycol solution was 3.35m/s. The inlet temperature of glycol solution changed according to mass velocity of refrigerant, to maintain certain degree of undercooling of outlet refrigerant. The research showed that the condensation heat transfer coefficient of both tubes increased with the increasing mass velocity of refrigerant. when the mass velocity of refrigerant increased from 300kg/m2s to 700kg/m2s, the condensation heat transfer coefficient in tube A was 1.87% to 6.28% higher than that of tube B. However, the flow resistance of the refrigerant in tube B was 9.56% to 11.05% higher than in tube A. The structure of tube A was superior to that of tube B.

Flow Patterns and Heat Transfer for Flow Boiling in Small to Micro Diameter Tubes

An overview of the recent developments in the study of flow patterns and boiling heat transfer in small to micro diameter tubes is presented. The latest results of a long-term study of flow boiling of R134a in five vertical stainless-steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1, and 0.52 mm are then discussed. During these experiments, the mass flux was varied from 100 to 700 kg/m2s and the heat flux from as low as 1.6 to 135 kW/m2. Five different pressures were studied, namely, 6, 8, 10, 12, and 14 bar. The flow regimes were observed at a glass section located directly at the exit of the heated test section. The range of diameters was chosen to investigate thresholds for macro, small, or micro tube characteristics. The heat transfer coefficients in tubes ranging from 4.26 mm down to 1.1 mm increased with heat flux and system pressure, but did not change with vapor quality for low quality values. At higher quality, the heat transfer coefficients decreased with increasing quality, indicating local transient dry-out, instead of increasing as expected in macro tubes. There was no significant difference between the characteristics and magnitude of the heat transfer coefficients in the 4.26 mm and 2.88 mm tubes but the coefficients in the 2.01 and 1.1 mm tubes were higher. Confined bubble flow was first observed in the 2.01 mm tube, which suggests that this size might be considered as a critical diameter to distinguish small from macro tubes. Further differences have now been observed in the 0.52 mm tube: A transitional wavy flow appeared over a significant range of quality/heat flux and dispersed flow was not observed. The heat transfer characteristics were also different from those in the larger tubes. The data fell into two groups that exhibited different influences of heat flux below and above a heat flux threshold. These differences, in both flow patterns and heat transfer, indicate a possible second change from small to micro behavior at diameters less than 1 mm for R134a.

Flow boiling in horizontal flattened tubes: Part II – Flow boiling heat transfer results and model

Experiments of flow boiling heat transfer were conducted in four horizontal flattened smooth copper tubes of two different heights of 2 and 3 mm. The equivalent diameters of the flattened tubes are 8.6, 7.17, 6.25, and 5.3 mm. The working fluids were R22 and R410A. The test conditions were: mass velocities from 150 to 500 kg/m 2 s, heat fluxes from 6 to 40 kW/m 2 and saturation temperature of 5 °C. The experimental heat transfer results are presented and the effects of mass flux, heat flux, and tube diameter on heat transfer are analyzed. Furthermore, the flow pattern based flow boiling heat transfer model of Wojtan et al. [L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes: Part I – A new diabatic two-phase flow pattern map, Int. J. Heat Mass Transfer 48 (2005) 2955–2969; L. Wojtan, T. Ursenbacker, J.R. Thome, Investigation of flow boiling in horizontal tubes: Part II – Development of a new heat transfer model for stratified-wavy, dryout and mist flow regimes, Int. J. Heat Mass Transfer 48 (2005) 2970–2985], using the equivalent diameters, were compared to the experimental data. The model predicts 71% of the entire database of R22 and R410A ±30% overall. The model predicts well the flattened tube heat transfer coefficients for R22 while it does not predicts well those for R410A. Based on several physical considerations, a modified flow boiling heat transfer model was proposed for the flattened tubes on the basis of the Wojtan et al. model and it predicts the flattened tube heat transfer database of R22 and R410A by 85.8% within ±30%. The modified model is applied to the reduced pressures up to 0.19.

G0501-5-1 凝縮器内における非凝縮性ガス挙動の数値シミュレーション(CFD)

To establish the computational method of non-condensable gas bahavior in condenser; numerical simulation of gas/gas two- phase flow (vapor/non-condensable gas) was carried out using two-fluid model. The amount of condensation of vapor was decided by overall heat transfer coefficient and temperature difference between vapor and heat exchanger tubes. The change of outside tube heat transfer coefficient by non-condensable gas density were considered. The results obtained from the proposed model showed that there was stagnantation of non-condensable gas at the bottom region of tube bundle, and the effect of vent tube length on the efficiency of condenser was figured out.

R05-(1) カーエアコン蒸発器における管内伝熱促進技術

High heat exchanger performance in Car air conditioners is very important to comfortable living and energy saving. We studied the refrigerant heat transfer enhancement in a car heat exchanger with projections (dimples) on its inner walls. Experimental and analytical results showed that elliptical dimple is optimum in heat transfer coefficient in for all types of dimple. Heat transfer coefficient of tubes with elliptical dimples is as twice that of a conventional tube with inner fins. For a evaporator tube with dimples, heat transfer maintained the same compared to a current evaporator with inner fined tubes however a weight to the evaporator with dimple was 10% less than current one.

Oscillation-controlled heat-transport tube (heat transfer coefficient in tubes in heating and cooling regions)

In the present study, heat transfer coefficients of oscillatory flow at inner surfaces of the heating and cooling regions of oscillation-controlled heat-transport tubes (OCHTs) are investigated numerically. The numerical simulation is conducted under the following three conditions for the tube walls at the heating and cooling regions: isothermal, extremely thin, and actual wall systems. Based on the numerical results and Hausen's correlating equation for laminar flow heat transfer in tubes, a correlating equation of the heat transfer coefficient is developed which can be generally applied to these three conditions. Next, using this correlating equation and the authors' simplified model of overall thermal resistance in OCHTs, heat-transport rates are predicted, and it is found that the predicted results are in good agreement with the numerical results. Finally, numerical simulation is conducted also to compare the heat-transport rates of OCHTs with those of the conventional forced circulation type under the same pumping power. The results indicate that there exist oscillatory flow regions in which the heat-transport rate of the phase-shifted OCHTs is larger than that of the conventional circulation type. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(6): 415–430, 1998

Augmentation of outside tube heat transfer coefficient during condensation of steam over horizontal copper tubes

An experimental investigation has been carried out to find the condensing side heat transfer coefficient, h o, during condensation of steam over a plain tube, a circular integral-fin tube(CIFT) and a spine integral-fin tube(SIFT). The CIFT and SIFT have enhanced the h o by a factor of 2.5 and 3.2 respectively. The experimental values of h o for CIFT were compared with those predicted from different models. The Honda and Nozu model underpredicted the values of h o in a range of 10 to 20 percent.

Oscillation-Controlled Heat Transport Tube. Heat Transfer Coefficient in Tubes at Heating and Cooling Regions.

In the present report, first, heat transfer coefficients of oscillatory flow at inner surfaces of the heating and cooling regions of oscillation controlled heat transport tubes (OCHTs) are investigated numerically. The numerical simulation is conducted under the following three conditions for the tube walls at the heating and cooling regions : isothermal, extremely thin and actual wall systems. Based on the numerical results and Hausen's correlating equation for laminar flow heat transfer in tubes, a correlating equation of the heat transfer coefficient is developed which can be generally applied to these three conditions. Next, using this correlating equation and the author's simplified model of overall thermal resistance in OCHTs, heat transport rates are predicted, and it is found that the predicted results are in good agreement with the numerical results. Finally, numerical simulation is conducted also to compare the heat transport rates of OCHTs with those of the conventional forced circulation type under the same pumping power. The results indicate that there exist oscillatory flow regions in which the heat transport rate of the phase shifted OCHTs is larger than that of the conventional circulation type.

How to obtain pool boiling heat transfer coefficients for tubes from wire experiments

Experiments for pool boiling are simpler when performed on horizontal wires than on horizontal tubes. It is shown how nucleate boiling results from wire experiments can be transformed into heat transfer coefficients for tubes. Thus, heat transfer coefficients can be obtained for tubes of 6 to 25 mm in diameter or 30 mm and larger (including horizontal flat plates) from experiments on a 0.1 mm thick platinum wire. Comparison with a large number of heat transfer coefficients from literature show good agreement with the equations introduced here.

The effect of solid particles on crossflow heat transfer in a tube array

An experimental investigation was made into the effects of particles on heat transfer in a staggered tube array at the third row where high heat transfer rates are due to high levels of turbulence. Systematic experiments were conducted with and without particles to determine the effects of particle size and concentration and of airflow rate on local heat transfer coefficients for tubes in an array. For all cases, the addition of the particles reduced the local Nusselt number at every measurement point. The suspension heat transfer is shown to be determined by a combination of thermal capacity effects and changes in the flow structure and turbulence levels within the array. The greatest reduction in Nusselt number was measured for the larger particles at the higher Reynolds number, but when potential enhancement due to increased thermal capacity is taken into account, the reduction in heat transfer due to turbulent suppression effects is more significant for the smaller particles. A preliminary analysis of the effect of the particles on the carrier-phase turbulence using a relative time hypothesis gives reasonable agreement with the observed variations in heat transfer when account is taken of the local thermal capacity effects.

Subcooled nucleate boiling from a horizontal multicylinder bundle in pool boiling

An experimental investigation was conducted to evaluate the effect of tube spacing and liquid temperature on the average and local heat transfer coefficients of tubes in a tube bundle. The dependency on these two parameters was found to be controlled by the flow field around the tubes.

Measurement of heat transfer coefficients in tubes by temperature oscillation analysis

AbstractAn experimental technique and evaluation method is described for the determination of local heat transfer coefficients in tubes or other ducts. By means of a rotary mixing valve, cold and warm fluid flows are mixed in order to generate an arbitrarily shaped but periodically oscillating inlet temperature profile at the test tube. The propagation of the fundamental harmonic oscillation from the fluid to the outer surface of the tube wall is calculated analytically. Comparison of fluid and wall oscillations yields the heat transfer coefficient to be measured. The inaccurate measurement of fluid bulk temperature in the centre of the cross‐section is compensated by an additional correction. Experiments were carried out with turbulent water flow through a copper tube. Measured heat transfer coefficients were compared to values calculated using Hausen's equation and good agreement was obtained.

Full-Core Test Method for Experimental Determination of Convective Boiling Heat Transfer Coefficients in Tubes of Crossflow Compact Evaporators

An experimental methodology is proposed in which localized convective boiling heat transfer coefficients inside the tubes of compact evaporators are determined by testing of full evaporator cores. The proposed technique makes use of a special test system having two main flow circuits. One of these flow loops is a conventional vapor compression system, which provides a steady, low-quality, two-phase flow of refrigerant to the tube side of the evaporator. The second primary flow loop provides a steady flow of the vapor of a second working fluid, which condenses on the finned side of the evaporator. Measured data from this system are analyzed using an iterative scheme. Trends in the variation of the refrigerant-side heat transfer coefficient determined by this method throughout a typical evaporator core are described, and the differences and similarities relative to previously published results for single round tubes are discussed.

Nonradiative Heat Transfer Coefficients and “Cold” Operating Experience With a Laboratory SCFBC Model

A cold test model study of the staged cascade fluidized bed coal combustion process is described. Proper cascade flow of solids through the system was found to be sensitive to bed and downcomer distributor plate pressure drops. Control of bed depth by weir level was also complicated at high fluidization velocities by the process of entrained particles falling back into the downcomer. Bed to tube heat transfer coefficients ranging from 142 to 256W/m2•K were measured at the top surface of a simulated boiler tube in shallow beds, 0.15 to 0.23m deep.

Local heat transfer and hot-spot factors in wire-wrap tube bundle

Heat transfer coefficients and hot-spot factors have been determined from measured local temperatures and calculated local mass flux in seven adjacent tubes and associated subchannels of a 61 wire-wrap tube bundle characteristic of the blanket of a GCFR (Gas Cooled Fast Reactor). The bundle consisted of 2.11 cm OD stainless steel tubes on a triangular array with a pitch/diameter ratio of P/ D = 1.05. The helical wire of 0.1067 cm in diameter was coiled on the tube with a respective initial orientation of 0–120–240°C and 30.48 cm helical pitch. The experiment used water at atmospheric pressure and temperature as coolant. The resulting dimensionless correlation for heat transfer is applicable to gases and all non-metal fluids in one phase flow when the fluid properties at subchannel bulk temperature are used. This correlation is based on local subchannel mass flux and is applicable to all wire-wrap configurations. Local subchannel mass fluxes were determined with a computer program COBRA IV and used to correlate the average Nusselt number for each subchannel in terms of local Reynolds number and fluid Prandtl number. The differences of up to 19% between that correlation and the one presented in earlier work are discussed in the text. The hot-spot factors on the convective heat transfer coefficient for tubes and subchannels are given as a function of Reynolds number based on a bundle average mass flux and a local subchannel hydraulic diameter. These factors are specific to the bundle configuration and are also dependent on the wire-wrap configuration.

Boiling on a finned tube and a finned tube bundle

An experimental investigation was carried out in a 151 dm 3 container in R11, at saturation state of 1 bar and 23.3°C. The heat transfer coefficients measured on a single tube in the container, resembled approximately results for the case that only one tube is heated within an in-line 18 tube bundle. For a twin tube arrangement, the heat transfer coefficient of the upper tube is up to 80% higher than that of the lower tube. The heat transfer curve of this upper tube represents quite well the heat transfer coefficients of tubes enclosed in the bundle. Only for the bottom and top tubes of the bundle differences do occur. The possibility to obtain tube bundle data, at least to some extent, by much simpler twin-tube or triple-tube experiments would facilitate experimental investigations.