Heat and mass transfer at rotary heat exchangers in consideration of the condensation potential

In many industrial processes as well as in air conditioning systems heat and moisture is transferred by rotary heat exchangers from the warm exhaust air flow to the cold supply air flow. Rotary heat exchangers are classified as sorption rotors, hygroscopic rotors and condensation rotors. Basic mechanisms of heat and moisture transfer are presented. By means of the condensation potential as the difference between the moisture content of the warm air flow and the moisture content of the cold air flow at saturation the humidity transfer at the different rotor types is investigated. The condensation potential as a reference parameter provides the possibility to describe the influence of various air conditions in exhaust air and supply air flow on the humidity transfer of different rotary heat exchangers and to compare these rotors with each other. In order to give an overview of relevant design parameters, the influence of the speed of turning, the flute height of the rotor matrix and the velocity of the air flow regarding the heat and mass transfer is considered.     

Intensity of convective mass/heat transfer in a rotary regenerator rotor with the transverse needle-fins

A forced convective mass transfer coefficient was electrochemically measured for a cylindrical bundle of transverse needle-fins ?1 × 10.9, applied as the rotor porous matrix of a rotary heat regenerator. The baffle inside the rotor was present. The technique based on the ferricyanide–ferrocyanide redox reaction controlled at the cathode, in the presence of a sodium hydroxide based electrolyte, was used in this experiment. A set of the six neighbouring fins, connected in parallel, was the cathode. The distribution of the mass transfer coefficient according to different static rotor angle position and the mean mass transfer Chilton–Colburn coefficient correlation j _M  = j _M (Re) for rotation numbers, Ro: 0, 0.8, 1.6 and 2.0 were stated in the mean Reynolds number, Re, range 180–985. The comparison was made between the convective heat fluxes of the pin-fins and the sheet rotor, for Ro = 0.     

Influence of non-uniform flow distribution on overall heat transfer in convective bundle of circulating fluidized bed boiler

In the paper the results of comparative investigations on heat transfer performance of boiler convective bundle and its additional surface, i.e. membrane water wall are presented. For this purpose the non-uniform flow field was modelled in an isothermal test stand operated in self-modeling mode. Then the heat transfer characteristics of such arrangement were estimated by means of naphthalene heat/mass transfer analogy technique. The bundle samples in the shape of round bars (rods) were cast with naphthalene and placed in 27 locations in the bundle while water-wall-modeling samples were coated with naphthalene by painting. Both types of samples were exposed to cold air flow. The results were then compared to the mean heat transfer performance of the same bundle exposed to uniform flow field. The differences of approximately 10% were noticed. Moreover, the heat transfer coefficients for additional surface were even three times lower than those of the bundle. In view of results obtained in the work, the commonly made assumption of equality of heat transfer coefficients for both the bundle and its additional surface may lead to certain errors in heat transfer calculations and discrepancies between the calculated values of heating surfaces area and later operational needs of steam generator.     

Experimental investigation of Marangoni condensation of ethanol–water mixture vapors on vertical tube

The heat transfer characteristics of the condensation of ethanol–water binary vapor on vertical tubes with the pipe diameter of 10 mm were investigated experimentally. The results showed that, with the change of the vapor-to-surface temperature difference, the condensation heat transfer coefficients revealed nonlinear characteristics with peak values under a wide variety of operating conditions. With the increasing pressure or velocity of the vapor, the heat transfer coefficients increased subsequently. The effect of vapor pressure or velocity on heat transfer coefficients reduced with the increasing ethanol mass fraction. It was noteworthy that, under low ethanol mass fractions (0.5–2%), the heat transfer coefficients augmented significantly, were about 5–8 times greater than that of pure steam. The comparison for different test blocks indicated that the condensation heat transfer coefficients for different pipe diameters were about the same value under the same operating condition. Significant heat transfer enhancement by Marangoni condensation could be achieved for full range of pipe diameter used in industrial condensers.     

Boiling heat transfer characteristics of nanofluids in a flat heat pipe evaporator with micro-grooved heating surface

An experimental study was performed to understand the nucleate boiling heat transfer of water–CuO nanoparticles suspension (nanofluids) at different operating pressures and different nanoparticle mass concentrations. The experimental apparatus is a miniature flat heat pipe (MFHP) with micro-grooved heat transfer surface of its evaporator. The experimental results indicate that the operating pressure has great influence on the nucleate boiling characteristics in the MFHP evaporator. The heat transfer coefficient and the critical heat flux (CHF) of nanofluids increase greatly with decreasing pressure as compared with those of water. The heat transfer coefficient and the CHF of nanofluids can increase about 25% and 50%, respectively, at atmospheric pressure whereas about 100% and 150%, respectively, at the pressure of 7.4 kPa. Nanoparticle mass concentration also has significant influence on the boiling heat transfer and the CHF of nanofluids. The heat transfer coefficient and the CHF increase slowly with the increase of the nanoparticle mass concentration at low concentration conditions. However, when the nanoparticle mass concentration is over 1.0 wt%, the CHF enhancement is close to a constant number and the heat transfer coefficient deteriorates. There exists an optimum mass concentration for nanofluids which corresponds to the maximum heat transfer enhancement and this optimum mass concentration is 1.0 wt% at all test pressures. The experiment confirmed that the boiling heat transfer characteristics of the MFHP evaporator can evidently be strengthened by using water/CuO nanofluids.     

Heat (mass) transfer and friction loss in two-pass ducts with various parallel rib arrangements

The present study investigated the effects of four parallel rib arrangements on heat/mass transfer and pressure drop characteristics in rotating two-pass ducts. The experiments of mass transfer and pressure measurement are performed to obtain heat transfer distributions and friction losses. The highest averaged heat (mass) transfer and friction loss in each region appeared in the turning region in the stationary cases, but appeared in the upstream region of the second pass in the rotating cases. Among the tested rib arrangements, the averaged heat (mass) transfer and the friction factor in the second pass in the stationary ducts were high in the cases with the NN- and PP-type ribs; however, in the rotating ducts, they were high in the cases with the NN- and PN-type ribs. The thermal performance in the four different rib arrangements was almost the same.     

Experimental investigation on heat transfer of forced convection condensation of ethanol–water vapor mixtures on a vertical mini-tube

In this paper, condensation heat transfer characteristics of ethanol–water vapor mixtures on a vertical mini-vertical tube with 1.221 mm outside diameter were investigated experimentally. The experiments were performed at different velocities and pressures over a wide range of ethanol mass fractions in vapor. The test results indicated that, with respect to the change of the vapor-to-surface temperature difference, the condensation curves of the heat transfer coefficients revealed nonlinear characteristics, and had peak values. At 2 % ethanol mass fraction in vapor, the condensation heat transfer coefficient value of the ethanol–water vapor mixture was found to have a maximum heat transfer coefficient of 50 kW m^?2 K^?1, which was 3–4 times than that of pure steam. The condensation heat transfer coefficients decreased with increased ethanol mass fraction in vapor. The vapor pressure and vapor velocity had a positive effect on the condensation heat transfer coefficients of ethanol–water vapor mixtures.     

Visualization and determination of local heat transfer coefficients in shell-and-tube heat exchangers for in-line tube arrangement by mass transfer measurements

The local heat transfer coefficients on the shell-side of shell-and-tube heat exchangers for in-line tube arrangement are visualized and determined from mass transfer measurements. The mass transfer experiments are carried out using a technique based on absorption, chemical and coupled colour reaction. Local mass transfer coefficients are measured for fully developed flow conditions on each tube surface. These coefficients were transformed to heat transfer coefficients by employing the analogy between heat and mass transfer. The averaged heat transfer coefficients and the pressure drop are compared with the predictions from the literature. Received on 2 May 1997     

Modelling for heat and mass transfer parameters in deep-frying of products

In this paper, the development of new models to determine heat and mass transfer parameters (HMTPs) in terms of the thermal diffusivity, heat transfer coefficient, moisture diffusivity and moisture transfer coefficient for slab, cylindrical, and spherical products being deep-fried was presented. In the model development, two cases of the Biot number, i.e., 0<Bi<100, and Bi≥100 in the transient heat and mass transfer were considered. In order to verify the models, frying experiments were performed using the cylindrically shaped potatoes as test samples, and these samples were fried in a deep fryer at 180°C. The lag factor and frying coefficient for a frying process, which affect heat and mass transfer parameters, were first-defined and considered most important process parameters. By using the temperature and moisture measurements, the frying coefficient and lag factor were determined and incorporated into the models. The HMTPs were determined in a simple and effective manner. In this respect, we can conclude that the present models are useful tools for determining the HMTPs for the products during frying and will be beneficial to the practical applications.     

Experimental study on heat transfer of thermally developing and developed, turbulent, horizontal pipe flow of dilute air-solids suspensions

Heat transfer characteristics of a turbulent, dilute air-solids suspension flow in thermally developing/developed regions were experimentally studied, using a uniformly heated, horizontal 54.5 mm-ID pipe and 43-μm-diameter glass beads. The local heat transfer was measured at 27 locations from the inlet to 120-dia downstream of the heated section over a range of Reynolds numbers 3×10⁴−1.2×10⁵ and solids loading ratio 0–3, and the fully developed profiles of air velocity/temperature and particle mass flux were measured at a location 140-dia downstream of the heated section using specially designed probes, inserted into the suspension flow. The effects of the Reynolds number, solids loading ratio, and azimuthal/longitudinal locations on the heat transfer characteristics and their interactions are discussed through comparison of the present results with the data obtained by other investigators. Received on 14 October 1996     

A GENERALIZED THERMOELASTIC DIFFUSION PROBLEM OF THIN PLATE HEATED BY THE ULTRASHORT LASER PULSES 1)

由于超短激光脉冲具有功率密度高、持续时间短、加工精度高等优势, 近年来被广泛应用于超精细加工、光学储存和微电子器件制造等领域. 本文基于L-S型广义热弹扩散理论, 建立了考虑材料记忆依赖效应和空间非局部效应的记忆依赖型非局部广义热弹扩散耦合理论, 它能够准确预测几何尺寸与内部特征尺寸相近结构的热弹扩散瞬态响应. 推导了所建理论的控制方程, 并基于拉普拉斯积分变换获得了控制方程的解. 作为算例, 利用所建理论和求解方法研究了半无限大薄板受非高斯激光脉冲加热和化学冲击联合作用下的热弹扩散瞬态响应问题, 得到了薄板的温度、化学势、位移、应力和浓度等随非局部参数、热时间迟滞因子和扩散时间迟滞因子等参数变化的分布规律. 结果表明: 传热对传质影响显著, 传质对传热影响甚微; 非局部参数对位移、应力影响显著, 对温度、化学势和浓度几乎没有影响. 该理论及求解方法的建立, 旨在实现材料在机械、热、化学势等冲击作用下传热传质瞬态响应的准确预测.     

超短激光脉冲加热薄板的广义热弹扩散问题

由于超短激光脉冲具有功率密度高、持续时间短、加工精度高等优势,近年来被广泛应用于超精细加工、光学储存和微电子器件制造等领域.本文基于L-S型广义热弹扩散理论,建立了考虑材料记忆依赖效应和空间非局部效应的记忆依赖型非局部广义热弹扩散耦合理论,它能够准确预测几何尺寸与内部特征尺寸相近结构的热弹扩散瞬态响应.推导了所建理论的控制方程,并基于拉普拉斯积分变换获得了控制方程的解.作为算例,利用所建理论和求解方法研究了半无限大薄板受非高斯激光脉冲加热和化学冲击联合作用下的热弹扩散瞬态响应问题,得到了薄板的温度、化学势、位移、应力和浓度等随非局部参数、热时间迟滞因子和扩散时间迟滞因子等参数变化的分布规律.结果表明:传热对传质影响显著,传质对传热影响甚微;非局部参数对位移、应力影响显著,对温度、化学势和浓度几乎没有影响.该理论及求解方法的建立,旨在实现材料在机械、热、化学势等冲击作用下传热传质瞬态响应的准确预测.     

Heat transfer and enhancement analyses of flow boiling for R417A and R22

The experimental study on heat transfer of R417A and R22 flow boiling inside a horizontal smooth and two internally grooved tubes with different geometrical parameters was conducted. Based on the experimental results, evaporation heat transfer characteristics of R417A and R22 flowing in different tubes, the influence of micro-fin geometrical parameters, vapor quality and mass flux of refrigerants on heat transfer enhancement factors, and the difference of enhancement factors between R417A and R22 were analyzed and discussed. The result indicates: whether for R22 or for R417A, the enhancement effect of Tube III having the narrower distance between micro-fins excels than Tube II. The influence of vapor qualities and mass fluxes on enhancement factors for R417A is different from R22. And the difference of enhancement factors between R417A and R22 appears different cases at different vapor quality regions.     

Local heat transfer in the first baffle compartment of the shell-and-tube heat exchangers for staggered tube arrangement

A mass transfer measuring method based on absorption, chemical and coupled colour reaction is used to visualize and determine the shell side local heat transfer in the first baffle compartment of shell-and-tube heat exchangers with segmental baffles for staggered tube arrangement. Local mass transfer coefficients were transformed into heat transfer coefficients by using the analogy between heat and mass transfer. The local, per-tube and integral heat transfer coefficient distributions are presented.     

Effects of cross ribs on heat/mass transfer in a two-pass rotating duct

Two-pass internal cooling passage with rib turbulators has been investigated for convective heat/mass transfer under rotating conditions. The flow and heat transfer characteristics in the cooling passage are very complicated so that it is required the detail analysis to design more efficient gas turbine blades. A naphthalene sublimation technique is employed to determine detailed local heat transfer coefficients using the heat and mass transfer analogy. The local heat/mass transfer and flow pattern in the cooling passage are changed significantly according to rib configurations, duct turning geometries and duct rotation speeds. Four different rib configurations are investigated to obtain the combined effects of the angled rib, duct turning and rotation. The results show that the duct rotation generates the heat/mass transfer discrepancy between the leading and trailing walls due to the secondary flows induced by the Coriolis force. The angled ribs generate a single rotating secondary flow with the cross-rib arrangement and the duct turning makes a strong Dean-type vortex. These vortices affect significantly the heat/mass transfer on the duct wall. The overall heat transfer pattern on the leading and trailing surfaces for the first and second passes are dependent on the duct rotation, but the local heat transfer trend is affected mainly by the rib arrangements. In addition, the present study observes the rotating effect in the two-pass smooth duct to obtain the baseline data in comparison with the ribbed duct for various rib arrangements.     

Flow boiling heat transfer and pressure drop of R-134a in a mini tube: an experimental investigation

This paper presents the results of an experimental study carried out with R-134a during flow boiling in a horizontal tube of 2.6 mm ID. The experimental tests included (i) heat fluxes in the range from 10 to 100 kW/m², (ii) the refrigerant mass velocities set to the discrete values in the range of 240-930 kg/(m² s) and (iii) saturation temperature of 12 and 22 °C. The study analyzed the heat transfer, through the local heat transfer coefficient along of flow, and pressure drop, under the variation of these different parameters. It was possible to observe the significant influence of heat flux in the heat transfer coefficient and mass velocity in the pressure drop, besides the effects of saturation temperature. In the low quality region, it was possible to observe a significant influence of heat flux on the heat transfer coefficient. In the high vapor quality region, for high mass velocities, this influence tended to vanish, and the coefficient decreased. The influence of mass velocity in the heat transfer coefficient was detected in most tests for a threshold value of vapor quality, which was higher as the heat flux increased. For higher heat flux the heat transfer coefficient was nearly independent of mass velocity. The frictional pressure drop increased with the increase in vapor quality and mass velocity. Predictive models for heat transfer coefficient in mini channels were evaluated and the calculated coefficient agreed well with measured data within a range 35% for saturation temperature of 22 °C. These results extend the ranges of heat fluxes and mass velocities beyond values available in literature, and add a substantial contribution to the comprehension of boiling heat transfer phenomena inside mini channels.     

Coupled heat and mass transfer in the entrance region of a circular tube with fully-developed parabolic flow and external convective heating

Coupled heat and mass transfer in the entrance region of a circular tube with fully-developed parabolic flow and external heating is analyzed. The closed form solutions of temperature and concentration distributions, local Nusselt numbers based on the total external heat transfer and convective heat transfer inside the tube, as well as the Sherwood number were obtained. The effects of Biot and Lewis number on heat and mass transfer were investigated.     

A rigorous analysis of simultaneous heat and mass transfer in the pasta drying process

This study presents a two dimensional analysis of coupled heat and mass transfer during the process of pasta drying. Velocity and temperature distributions of air flowing around the pasta are predicted in steady state condition. Using these profiles and the similarity between heat and mass boundary layers, local convective heat and mass transfer coefficients were determined on different points of pasta surface. By employing these values, the solution of coupled heat and mass transfer equations within the pasta object in unsteady state condition was obtained. Furthermore the effects of operating conditions such as velocity, temperature and relative humidity of air flow on drying rate of pasta were studied. Sensitivity analysis results show that the effects of air temperature and relative humidity on the rate of drying are more important than the effect of air velocity. Finally, the results obtained from this analysis were compared with the experimental data reported in the literatures and a good agreement was observed while, no adjustable parameter is used in the presented model.     

Heat and mass transfer of a water film falling down a tilted plate with radiant heating and evaporation

This paper has concerned the heat and mass transfer of a water film falling down a tilted plate with radiant heating and water evaporation. A cluster of physical models was developed for evaluating the properties of heat and mass transfer. A fully implicit control-volume finite-difference procedure was used to solve the coupling equations. The effects of various parameters on heat and mass transfer were investigated. The results showed that the mass fraction of water vapor in ambient atmosphere and the flow turbulence played key roles in the heat and mass transfer. The ambient atmospheric temperature dramatically affected the sensible heat flux. However its effect on the latent heat flux is negligibly small. The magnitude of solar incident flux had an intense influence on the water film temperature. Received on 29 January 1998     

Effects of Lewis number on coupled heat and mass transfer in a circular tube subjected to external convective heating

Heat and mass transfer in a circular tube subject to the boundary condition of the third kind is investigated. The closed form of temperature and concentration distributions, the local Nusselt number based on the total external heat transfer and convective heat transfer inside the tube, as well as the Sherwood number were obtained. The effects of Lewis number and Biot number on heat and mass transfer were investigated.