Heat transfer of air-water dispersed flow in a vertical pipe

Heat transfer of air-water dispersed flow in a vertical heating pipe and its enhancement have been studied. The axial and circumferential wall temperature distributions were measured using various mist ratios and wall heat fluxes. The measured wall temperature increased sharply at a particular streamwise location, with a notable variation in the circumferential profile. This sharp increase was conceivably caused by a breakdown of the water film rather than by its dryout. A separate unheated experiment was carried out to estimate the droplet deposition velocity and the water-film flow rate. A numerical analysis, taking into account heat and mass transfer from the water film to the bulk flow, was performed in order to estimate the mean wall temperature. Good agreement was obtained with the experimental results in the area where the entire inner surface of the pipe was covered with the water film. In this area, the rate of heat transfer was approximately seven times larger than that for single phase air flow. This enhancement was shown to be due mainly to evaporation of the water film. The mechanism of heat transfer enhancement is discussed in detail using the numerical analysis results. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 255–270, 1998     

Numerical analysis of heat transfer in pulsating turbulent flow in a pipe

Convection heat transfer in pulsating turbulent flow with large velocity oscillating amplitudes in a pipe at constant wall temperature is numerically studied. A low-Reynolds-number (LRN) k–ε turbulent model is used in the turbulence modeling. The model analysis indicates that Womersley number is a very important parameter in the study of pulsating flow and heat transfer. Flow and heat transfer in a wide range of process parameters are investigated to reveal the velocity and temperature characteristics of the flow. The numerical calculation results show that in a pulsating turbulent flow there is an optimum Womersley number at which heat transfer is maximally enhanced. Both larger amplitude of velocity oscillation and flow reversal in the pulsating turbulent flow also greatly promote the heat transfer enhancement.     

Spectral measurements of turbulent heat and momentum transfer in fully developed pipe flow

Cross-spectra of turbulent heat and momentum transfer in fully developed pipe flow of air are used to examine the similarity between the two processes. Results taken at a Reynolds number (based on pipe radius and centre line velocity) of 34 700 indicate that the low wavenumber components which in a previous study showed a high correlation between heat and momentum, are very ineffective in the transfer of these quantities from one fluid layer to the next in the wall region but become increasingly effective in the core flow. The mechanisms of the two transfer processes as measured by the spectral cross-correlation coefficients are found to be very similar in the energy containing range of wavenumbers. Local isotropy is approached by the velocity field at lower wavenumbers than by the temperature field.     

Heat transfer analysis of stratified flow in rotating heat pipes with cylindrical and stepped walls

This paper deals with the flow behavior and the related heat transfer characteristics of stratified flow in axially rotating heat pipes with cylindrical and stepped wall configurations. Flow patterns are presented with existing experimental data of heat transfer in cylindrical and stepped wall rotating heat pipes. Theoretical and semi-empirical models for calculation of the condensation and evaporation heat transfer coefficients are developed. Key dimensionless numbers such as Froude, Galileo, G and ξ-number are identified. Existing experimental data from a rotating cylindrical heat pipe are analyzed and used for regression based on semi-empirical models. Good agreement between the predicted results and experimental data was obtained. Comparison between the present heat transfer models rotating cylindrical wall heat pipes and experimental data from a stepped wall heat pipe shows that the present models can be used to predict the condensation and evaporation heat transfer coefficients in a rotating stepped wall heat pipe.     

Experimental and theoretical studies of thermal conductivity,viscosity and heat transfer coefficient of titania and alumina nanofluids

Thermal conductivity, viscosity and heat transfer coefficient of water-based alumina and titania nanofluids have been investigated. The thermal conductivity of alumina nanofluids follow the prediction of Maxwell model, whilst that of titania nanofluids is slightly lower than model prediction because of high concentration of stabilisers. None of investigated nanofluids show anomalously high thermal conductivity enhancement frequently reported in literature. The viscosity of alumina and titania nanofluids was higher than the prediction of Einstein–Batchelor model due to aggregation. Heat transfer coefficients measured in nanofluids flowing through the straight pipes are in a very good agreement with heat transfer coefficients predicted from classical correlation developed for simple fluids. Experimental heat transfer coefficients in both nanofluids as well as corresponding wall temperatures agree within ±10% with the values obtained from numerical simulations employing homogeneous flow model with effective thermo-physical properties of nanofluids. These results clearly shows that titania and alumina nano-fluids do not show unusual enhancement of thermal conductivity nor heat transfer coefficients in pipe flow frequently reported in literature.     

Heat transfer performance and exergetic optimization for solar receiver pipe

The basic physical model of solar receiver pipe with solar selective coating is established, and associated heat transfer and exergetic performances are analyzed and optimized. Because of the heat losses of natural convection and infrared radiation, the energy absorption efficiency has a maximum at optimal incident energy flux. As the pipe radius decreases or flow velocity rises, the wall temperature drops for higher heat transfer coefficient, while the heat absorption efficiency increases. Along the flow direction, the heat absorption efficiency almost linearly decreases, while the exergetic efficiency will first increase and then decrease. As the inlet temperature rises, the heat absorption efficiency of the solar receiver pipe decreases, while the exergetic efficiency of absorbed energy obviously increases, so the exergetic efficiency of incident energy will reach maximum at the optimal inlet temperature. Additionally, the maximum exergetic efficiency of incident energy and optimal inlet temperature both increase with flow velocity.     

Velocity and temperature profiles in a wall jet

Conditions approximating those of the wall jet have been obtained by operating a system, involving tangential air injection into a turbulent boundary layer, with very low values of the free stream velocity. Under such conditions the flow is essentially produced by the injected air and measurement of the velocity profiles shows correspondence to the theory for the turbulent wall jet. With some alternation of the eddy diffusivity of that theory the measured temperature profiles can also be predicted and these, together with the velocity profiles, are shown to agree generally with the measured values of the adiabatic wall temperature. The measured heat transfer coefficients are related to the hydrodynamic characteristics by a formulation of the Colburn type.     

Flow and heat transfer characteristics in pulsating pipe flows (effects of pulsation on internal heat transfer in a circular pipe flow)

Effects of pulsation on flow and heat transfer characteristics are experimentally examined in the pulsating pipe flows having sinusoidal velocity fluctuations around a nonzero mean. By systematically varying three pulsation parameters (the amplitude, frequency, and mean velocity), time-averaged and fluctuating temperature profiles are measured under the heating condition of constant wall temperature using saturated vapor. The mean Nusselt number, Nu_p, is calculated, and compared with that in ordinary turbulent pipe flows without pulsation. The results show that Nu_p, decreases initially as the pulsation amplitude increases, then recovers gradually, and finally becomes much greater than the original value. In pulsating pipe flows with a nonzero mean velocity, therefore, pulsation cannot always promote heat transfer, but sometimes suppresses it, depending mainly on the pulsation amplitude and mean velocity. It is also found that these heat transfer characteristics of a pulsating pipe flow are controlled by the transition of flow patterns with pulsation amplitude from a fully turbulent flow to a conditionally turbulent flow via a transitional flow. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(5): 323–341, 1996     

Thermal‐fluid transport phenomena in an axially rotating flow passage with twin concentric orifices of different radii

This paper investigates the thermal fluid‐flow transport phenomena in an axially rotating passage in which twin concentric orifices of different radii are installed. Emphasis is placed on the effects of pipe rotation and orifice configuration on the flow and thermal fields, i.e. both the formation of vena contracta and the heat‐transfer performance behind each orifice. The governing equations are discretized by means of a finite‐difference technique and numerically solved for the distributions of velocity vector and fluid temperature subject to constant wall temperature and uniform inlet velocity and fluid temperature. It is found that: (i) for a laminar flow through twin concentric orifices in a pipe, axial pipe rotation causes the vena contracta in the orifice to stretch, resulting in an amplification of heat‐transfer performance in the downstream region behind the rear orifice, (ii) simultaneously the heat transfer rate in the area between twin orifice is intensified by pipe rotation, (iii) the amplification of heat transfer performance is affected by the front and rear orifice heights. Results may find applications in automotive and rotating hydraulic transmission lines and in aircraft gas turbine engines. Copyright © 2005 John Wiley & Sons, Ltd.     

理想流体对流传热问题的理论解

研究理想流体受迫对流传热和自然对流传热问题的理论解。采用流体无垂直于壁面法线方向运动(即无穿透)的条件取代黏性流体在壁面无滑移条件，解决了流体在边界上有滑移时计算对流传热系数的困难，给出了理想流体与平壁受迫对流传热、理想流体与竖直壁面自然对流传热和理想流体在管内受迫对流传热的理论解。结果表明：理想流体的对流传热与黏性流体同样存在着热边界层。在外部流动的情况下，无论受迫对流传热还是自然对流传热，对流传热系数都与流体的导热系数、密度和比热三乘积的二分之一次方成正比。在管内受迫对流的情况下，当无因次长度大于0．05时，局部Nu和界面无因次温度分布都不再变化，对于恒热流边界条件，Nu等于8，截面无因次平均温度等于2；对于恒壁温边界条件，Nu等于5．782，截面无因次平均温度等于2．316。     

Heat transfer of a non-Newtonian fluid (Carbopol aqueous solution) in transitional pipe flow

An experimental study of the forced convection heat transfer for non-Newtonian fluid flow in a pipe is presented. We focus particularly on the transitional regime. A wall boundary heating condition of heat flux is imposed. The non-Newtonian fluid used is Carbopol (polyacrylic acid) aqueous solutions. Detailed rheology as well as the variation of the rheological parameters with temperature are reported. Newtonian and shear thinning fluids are also tested for comparative purposes. The characterization of the flow and the thermal convection is made via the pressure drop and the wall temperature measurements over a range of Reynolds number from laminar to turbulent regime. Our measurements show that the non-Newtonian character stabilizes the flow, i.e., the critical Reynolds number to transitional flow increases with shear thinning and yield stress. The heat transfer coefficients are given and compared with heat transfer laws for different regime flows. Details when the heat transfer coefficient loses rapidly its local dependence on the Reynolds number are analyzed.     

Investigation of local heat transfer coefficients in plate heat exchangers with temperature oscillation IR thermography and CFD

A method for the measurement of local convective heat transfer coefficients from the outside of a heat-transferring wall has been developed. This method is contact-free and fluid independent, employing radiant heating by laser or halogen spotlights and an IR camera for surface temperature measurements; it allows for the rapid evaluation of the heat transfer coefficient distribution of sizable heat exchanger areas. The technique relies first on experimental data of the phase lag of the outer surface temperature response to periodic heating, and second on a simplified numerical model of the heat exchanger wall to compute the local heat transfer coefficients from the processed data. The IR temperature data processing includes an algorithm for temperature drift compensation, phase synchronization between the periodic heat flux and the measured temperatures, and Single Frequency Discrete Fourier Transformations. The ill-posed inverse heat conduction problem of deriving a surface map of heat transfer coefficients from the phase-lag data is solved with a complex number finite-difference method applied to the heat exchanger wall. The relation between the local and the mean heat transfer coefficients is illuminated, calculation procedures based on the thermal boundary conditions are given. The results from measurements on a plate heat exchanger are presented, along with measurements conducted on pipe flow for validation. The results show high-resolution surface maps of the heat transfer coefficients for a chevron-type plate for three turbulent Reynolds numbers, including a promising approach of visualizing the flow field of the entire plate. The area-integrated values agree well with literature data. CFD calculations with an SST and an EASM–RSM were carried out on a section of a PHE channel. A comparison with the measured data indicates the shortcomings of even advanced turbulence models for the prediction of heat transfer coefficients but confirms the advantages of EASM–RSM in complex flows.     

A numerical study to investigate the effect of turbulators on thermal flow and heat performance of a 3D pipe

To reduce the heat exchanger's costs in a highly competitive industry, thermal performance enhancement of the heat exchangers has successfully gained attention in the last few decades. Among different engineering approaches, the application of the enhanced pipes provides a key solution to improve heat performance. In this paper, the investigation develops a numerical study based on the commercially available computational fluid dynamics codes on the turbulent flow in three-dimensional tubular pipes. Various concavity (dimple) diameters with corrugation and twisted tape configurations are investigated. The study has shown that perforated geometrical parameters lead to a high fluid mixing and flow perturbation between the pipe core region and the walls, hence better thermal efficiency. Moreover, a model of concavity (dimple) with a 4 mm diameter allows the highest heat transfer enhancement among other designs. In addition, the study shows that due to the disturbance between the pipe core region and the pipe wall, the transverse vortices and swirl flow generated are forceful, which leads to better heat transfer enhancement compared with the conventional (smooth) pipes. As the Reynolds number (Re) rises, the mixing flow, secondary, and separation flow extend to become higher than the values in a smooth pipe, allowing a higher value of performance evaluation factor to be achieved for a dimple diameter of 1mm at the low Re values. This study, therefore, shows the promising potential of the enhanced pipes in the heat transfer enhancement of heat exchangers that is crucial in industrial applications to save more energy.     

Heat Transfer Enhancement in a Pipe Using a Flow Inverter

Flow inversion, transferring a liquid from the wall region toward the center of the pipe or from the axis toward the heat transfer surface, improves heat transfer in the laminar flow regime. While a fully developed velocity profile is quickly established, a thin thermal boundary layer is preserved for a considerable distance in the pipe behind the flow inverter for highly viscous liquids. Thus the pressure drop is increased only locally (by the inverter itself), while heat transfer enhancement is also seen in a long straight section of the pipe. Two original flow inverter designs were tested in a long pipe (3 m in length) heated by condensing steam, using starch molasses as a working medium. Experiments carried out in the range of Reynolds number 4–60 and Graetz number 150–700 resulted in an increase of 20–35% in the heat transfer coefficient, accompanied by a 30–40% increase in the pressure drop. The experimental results confirm the numerical model prediction (within an extended Reynolds number range starting from 0.1). Thus an almost 40% increase in heat transfer can be expected at the optimal Graetz number in the range 50–100, using only one flow inverter located in the middle of a sufficiently long pipe.     

The effect of liquid film evaporation on flow boiling heat transfer in a micro tube

Flow boiling in micro channels is attracting large attention since it leads to large heat transfer area per unit volume. Generated vapor bubbles in micro channels are elongated due to the restriction of channel wall, and thus slug flow becomes one of the main flow regimes. In slug flow, sequential bubbles are confined by the liquid slugs, and thin liquid film is formed between tube wall and bubble. Liquid film evaporation is one of the main heat transfer mechanisms in micro channels and liquid film thickness is a very important parameter which determines heat transfer coefficient. In the present study, liquid film thickness is measured by laser focus displacement meter under flow boiling condition and compared with the correlation proposed for an adiabatic flow. The relationship between liquid film thickness and heat transfer coefficient is also investigated. Initial liquid film thickness under flow boiling condition can be predicted well by the correlation proposed under adiabatic condition. Under flow boiling condition, liquid film surface fluctuates due to high vapor velocity and shows periodic pattern against time. Frequency of periodic pattern increases with heat flux. At low quality, heat transfer coefficients calculated from measured liquid film thickness show good accordance with heat transfer coefficients obtained directly from wall temperature measurements.     

Heat transfer performance of an external receiver pipe under unilateral concentrated solar radiation

The heat transfer and absorption characteristics of an external receiver pipe under unilateral concentrated solar radiation are theoretically investigated. Since the heat loss ratio of the infrared radiation has maximum at moderate energy flux, the heat absorption efficiency will first increase and then decrease with the incident energy flux. The local absorption efficiency will increase with the flow velocity, while the wall temperature drops quickly. Because of the unilateral concentrated solar radiation and different incident angle, the heat transfer is uneven along the circumference. Near the perpendicularly incident region, the wall temperature and absorption efficiency slowly approaches to the maximum, while the absorption efficiency sharply drops near the parallelly incident region. The calculation results show that the heat transfer parameters calculated from the average incident energy flux have a good agreement with the average values of the circumference under different boundary conditions. For the whole pipe with coating of Pyromark, the absorption efficiency of the main region is above 85%, and only the absorption efficiency near the parallelly incident region is below 80%. In general, the absorption efficiency of the whole pipe increases with flow velocity rising and pipe length decreasing, and it approaches to the maximum at optimal concentrated solar flux.     

Forced convective mixing in a zero boil-off cryogenic storage tank

This paper presents the results of a study of fluid flow and heat transfer of liquid hydrogen in a cryogenic storage tank with a heat pipe and an array of pump-nozzle units. A forced flow is directed onto the evaporator section of the heat pipe to prevent the liquid from boiling off when heat leaks through the tank wall insulation from the surroundings. An axisymmetric computational model was developed for the simulation of convective heat transfer in the system. Steady-state velocity and temperature fields were solved from this model by using the finite element method. Forty five configurations of geometry and velocity were considered. As the nozzle fluid speed increases, the values of the maximum, average, and spatial standard deviation of the temperature field decrease nonlinearly. Parametric analysis indicates that overall thermal performance of the system can be significantly improved by reducing the gap between the nozzle and the heat pipe, while maintaining the same fluid speed exiting the nozzle. It is also indicated that increased inlet tube length of the pump-nozzle unit results in slightly better thermal performance. Increased heat pipe length also improves thermal performance but only for low fluid speed.     

Experimental investigation of coupled conduction and laminar convection in a circular tube

Wall heat conduction effects on laminar flow heat transfer are experimentally investigated. The steady flow of water through a uniformly heated copper pipe is considered in the experiment, which covers a range of Reynolds numbers from 500 to 1900. The thermal behaviour of the test section is simulated numerically and the influence of conduction along the pipe wall is therefore accounted for in the reduction of the data. Fully developed flow results satisfactorily compare with predictions by a theoretical method previously developed by the authors [Heat Technol. 2,72 (1984)]. Results are also reported for the case where the velocity profile is partially developed at the inlet of the heat transfer section. The combined effects on heat transfer of flow development and of wall axial heat conduction are discussed.     

Heat transfer coefficients for drying in pulsating flows

Pulsating flows generated by a Rijke type combustor are studied for drying of grains and food particles. It is assumed that the velocity fluctuations are the main factor in the enhancement of the drying process. The heat transfer coefficients for drying in vibrating beds are utilized to estimate the heat transfer coefficients of fixed beds in pulsating and permeating flows and are compared to the steady flow heat transfer coefficients obtained for solid porous bodies, after perturbing the main flow. The cases considered are compared to the convective heat transfer coefficients employed in non-pulsating drying.     

聚光太阳能吸热管的吸热传热特性

基于太阳能选择性吸收涂层的辐射性能,建立聚光太阳能吸热管光热耦合传输的数理模型,理论研究聚光太阳能吸热管的吸热传热特性。研究表明,吸热管壁温度随着聚光能流密度增加而线性升高,而吸热效率在中等聚光能流密度时达最大值。太阳能选择性吸收涂层性能对吸热传热有重要影响,具有低红外发射系数涂层的系统吸热效率明显较高,而红外辐射能量损失率则在中等聚光能流密度时最小。管内强迫对流可以显著提高吸热管效能,吸热效率随流速增加而提高,而管壁温度则显著下降。