Pulsating flow in a heat exchanger

The problem of heat transfer in industrial processes, heat exchangers, and combustion chambers is formulated for a case where flow inside the chamber consists of a periodic motion imposed on a fully developed turbulent flow. It is shown that the velocity pulsations induce harmonic oscillations in temperature, thus breaking the temperature field into a steady mean part and a harmonic part. The interaction between the velocity and temperature oscillations introduces an extra term into the energy equation which reflects the effect of pulsations in producing higher heat transfer rates. The analysis shows that when the mean temperature is fully developed with constant heat flux at the wall, there is no effect of the velocity pulsations on the total heat transfer rate along the chamber. For the case where the mean temperature profile is not fully developed, analytical solutions are obtained for asymptotic values of the pulsations frequency. The results show the temperature gradient and its dependence on the frequency. These results are used to evaluate the feasibility of pulsating the flow in a heat exchanger for obtaining higher rates of heat transfer.     

Warm up of an automotive catalyst substrate by pulsating flow: a single channel modelling approach

Rapid warm up of an automotive catalyst substrate is important for early light off. This work considers the results from a model of warm up in a single channel. The mass flow is pulsating with high amplitude, about 75% of mean flow, but without flow reversal. The flow regime is laminar within the channel. Pulsations occur with frequency in the range 16–100 Hz, and are important in close-coupled systems where the catalyst is located near to the engine and where the rate of rise of gas inlet temperature with time is rapid, about 15 K/s. The use of a single channel model with conjugate heat transfer enables the heat transfer coefficient to be evaluated and compared with results from steady flow simulations. The value of the augmentation factor based on heat flux is found to be less than unity. The value of the augmentation factor based on heat transfer coefficient depends on the method for calculating the mean heat transfer coefficient, but is generally less than unity. The changes caused by pulsations will be small in practical systems. Changes in wall temperature found in the simulations are the result of the cumulative effect of changes in the mass flow rate.     

Influence of the position and number of decay heat exchangers on the thermal hydraulics of a slab test facility A comparison of analytical and experimental data

To assess the capability of passive decay heat removal systems of an advanced pool-type liquid-metal cooled reactor, natural circulation experiments have been performed to investigate the in-vessel cooling modes caused by the position and number of decay heat exchangers in operation. The rather simple slab test facility AQUARIUS is equipped with an electrically heated core and decay heat exchangers. Four different arrangements of heat exchangers are under consideration to study the temperature distribution and the flow behavior in the apparatus during both symmetrical and asymmetrical heat removal. The experiments have been carried out in water under laminar flow conditions.

Temperatures have been measured under quasi-steady-state conditions. The observed flow paths have been documented photographically. In case of asymmetrical heat removal, especially when a single heat exchanger is operated in one of both upper plena, the temperature distribution and the flow behavior are different from symmetrical cooling modes. A comparison of analytically predicted temperatures using the COMMIX-2(V) computer program with experimental data shows reasonably matches the findings. The results of the numerically determined velocity fields are in good agreement with the visual observations.     

Numerical solutions of pulsating flow and heat transfer characteristics in a pipe

Numerical studies are made of flow and heat transfer characteristics of a pulsating flow in a pipe. Complete time-dependent laminar boundary-layer equations are solved numerically over broad ranges of the parameter spaces, i.e., the frequency parameter β and the amplitude of oscillation A. Recently developed numerical solution procedures for unsteady boundary-layer equations are utilized. The capabilities of the present numerical model are satisfactorily tested by comparing the instantaenous axial velocities with the existing data in various parameters. The time-mean axial velocity profiles are substantially unaffected by the changes in β and A. For high frequencies, the prominent effect of pulsations is felt principally in a thin layer near the solid wall. Skin friction is generally greateer than that of a steady flow. The influence of oscillation on skin friction is appreciable both in terms of magnitude and phase relation. Numerical results for temperature are analyzed to reveal significant heat transfer characteristics. In the downstream fully established region, the Nusselt number either increases or decreases over the steady-flow value, depending on the frequency parameter, although the deviations from the steady values are rather small in magnitude for the parameter ranges computed. The Nusselt number trend is amplified as A increases and when the Prandtl number is low below unity. These heat transfer characteristics are qualitatively consistent with previous theoretical predictions.     

Forced convection with laminar pulsating flow in a tube

The heat transfer model of laminar pulsating flow in a tube in rolling motion is established. The correlations of velocity, temperature and Nusselt number are obtained. The effects of several parameters on Nusselt number are investigated. The theoretical results are consistent with experimental data. Then the results are evaluated with Nield and Kuznetsov’s results. It is found that Nield and Kuznetsov’s results are not applicable for the laminar pulsating flow in nuclear power systems in ocean environments.     

Effect of flow pulsation on laminar heat transfer between two parallel plates

The problem of heat transfer in viscous laminar pulsatile flow between two parallel plates is solved by means of a finite difference method. Boundary conditions of constant wall temperature and constant wall heat flux are considered separately. The numerical results show that flow pulsations change the instantaneous Nusselt number, but do not have any significant effect on the time-averaged values. A trend in reduction of timeaveraged Nusselt number is observed when the amplitude of flow pulsation increases and the frequency decreases. The validity of the result is limited to the case when no flow reversal exists.     

Study on transient local entropy generation in pulsating fully developed laminar flow through an externally heated pipe

This study presents the investigation of transient local entropy generation rate in pulsating fully developed laminar flow through an externally heated pipe. The flow inlet to the pipe is considered as pulsating at a constant period and amplitude (only the velocity oscillates). The simulations are extended to include different pulsating flow cases (sinusoidal flow, step flow, and saw-down flow). To determine the effects of the mean velocity, the period and the amplitude of the pulsating flow on the entropy generation rate, the pulsating flow is examined for various cases of these parameters. Two-dimensional flow and temperature fields are computed numerically with the help of the fluent computational fluid dynamics (CFD) code. In addition to this CFD code, a computer program has been developed to calculate numerically the entropy generation and other thermodynamic parameters by using the results of the calculations performed for the flow and temperature fields. In all investigated cases, the irreversibility due to the heat transfer dominates. The step flow constitutes the highest temperature (about 919 K) and generates the highest total entropy rate (about 0.033 W/K) within the pipe. The results of this study indicate that in the considered situations, the inverse of square of temperature (1/T ²) is more dominant on the entropy generation than the temperature gradients, and that the increase of the mean velocity of the pulsating flow has an adverse effect on the ratio of the useful energy transfer rate to irreversibility rate.     

A simplified model of heat transfer in heat exchangers and stack plates of thermoacoustic refrigerators

A simplified model of heat transfer was developed to investigate the thermal behavior of heat exchangers and stack plates of thermoacoustic devices. The model took advantage of previous results describing the thermal behavior of the thermoacoustic core and heat transfer in oscillating flow to study the performance of heat exchangers attached to the core. The configuration considered is a flat tube (with a working fluid flowing in the tube) of the thickness of the stack plate attached to both ends of the stack plate. Geometrical and operational parameters as well as thermophysical properties of the heat exchangers, transport fluids in the heat exchangers, stack plate and the thermoacoustic working fluid were organized into dimensionless groups that allowed accounting for their impact on the performance of the heat exchangers. Two types of thermal boundary conditions were considered: constant temperature and constant heat flux along the heat exchanger tubes. Numerical simulations were carried out with the model introduced in the paper. The temperature distributions and heat fluxes near the edge of the stack plate were found to be nonlinear. The influence of system parameters on the thermal performance of the heat exchangers was analyzed.This article is dedicated to Prof. D. Mewes, whose knowledge, creativity, enthusiasm and dedication to engineering science was an inspiration to me and to many students, scientists, engineers and colleagues all over the world (C. Herman).     

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.     

Characteristics of low-temperature short heat pipes with a nozzle-shaped vapor channel

This paper presents the results of experimental and numerical studies of heat transfer and swirling pulsating flows in short low-temperature heat pipes whose vapor channels have the form of a conical nozzle. It has been found that as the evaporator of the heat pipe is heated, pressure pulsations occur in the vapor channel starting at a certain threshold value of the heat power, which is due to the start of boiling in the evaporator. The frequency of the pulsations has been measured, and their dependence on the superheat of the evaporator has been determined. It has been found that in heat pipes with a conical vapor channel, pulsations occur at lower evaporator superheats and the pulsation frequency is greater than in heat pipes of the same size with a standard cylindrical vapor channel. It has been shown that the curve of the heat-transfer coefficient versus thermal load on the evaporator has an inflection corresponding to the start of boiling in the capillary porous evaporator of the heat pipe.     

Pulsating flows in heat exchangers — an experimental study

The effect of pulsating flows on the performance of a heat exchanger is studied experimentally. The experiments are conducted in a steam-water, double pipe heat exchanger for 500相似文献   

Convective heat transfer characteristics of high-pressure gas in heat exchanger with membrane helical coils and membrane serpentine tubes

Since the heat transfer performance of syngas cooler affects the efficiency of the power generating system with integrated coal gasification combined cycle (IGCC) directly, it is important to obtain the heat transfer characteristics of high-pressure syngas in the cooler. Heat transfer in convection cooling section of pressurized coal gasifier with the membrane helical coils and membrane serpentine tubes under high pressure is experimentally investigated. High pressure single gas (He or N₂) and their mixture (He + N₂) gas serve as the test media in the test pressure range from 0.5 MPa to 3.0 MPa. The results show that the convection heat transfer coefficient of high pressure gas is influenced by the working pressure, gas composition and symmetry of flow around the coil, of which the working pressure is the most significant factor. The average convection heat transfer coefficients for various gases in heat exchangers are systematically analyzed, and the correlations between Nu and Re for two kinds of membrane heat exchangers are obtained. The heat transfer coefficient of heat exchanger with membrane helical coils is greater than that of the membrane serpentine-tube heat exchanger under the same conditions. The heat transfer coefficient increment of the membrane helical-coil heat exchanger is greater than that of the membrane serpentine-tube heat exchanger with the increase of gas pressure and velocity.     

Heat transfer and heat transfer fouling in Kraft black liquor evaporators

A large number of experiments have been performed with New Zealand Forest Products Kraft black liquor to measure heat transfer coefficients and fouling rates during convective and subcooled flow boiling heat transfer as a function of surface temperature, bulk temperature, velocity, and solids concentration. Results from experiments with two chemical fouling inhibitors, with Teflon surface coating and in plate and frame heat exchangers, also are presented. The fouling deposits are analyzed with respect to appearance, composition, and process conditions for which they were obtained. With the assumption of chemical reaction-controlled fouling, a deposition model is developed and compared with the experimental data.     

Boundary layer in the vicinity of the stagnation point of a body of axial symmetry in a turbulent stream

The flow in the boundary layer in the vicinity of the stagnation point of a flat plate is examined. The outer stream consists of turbulent flow of the jet type, directed normally to the plate. Assumptions concerning the connection between the pulsations in velocity and temperature in the boundary layer and the average parameters chosen on the basis of experimental data made it possible to obtain an isomorphic solution of the boundary layer equations. Equations are obtained for the friction and heat transfer at the wall in the region of gradient flow taking into account the effect of the turbulence of the impinging stream. It is shown that the friction at the wall is insensitive to the turbulence of the impinging stream, while the heat transfer is significantly increased with an increase in the pulsations of the outer flow. These properties are confirmed by the results of experimental studies [1–4].Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 83–87, September–October, 1973.     

Numerical study of heat transfer enhancement of finned flat tube bank fin with vortex generators mounted on both surfaces of the fin

Tube bank fin heat exchanger is one of the most compact heat exchangers, and it is widely used in industry equipments. The flat tube bank fin heat exchangers with vortex generators (VGs) have significant good heat transfer performance, and are used as radiators of locomotive. Here, we study heat transfer enhancement of a new fin where VGs are mounted on both surfaces of the fin. The heat transfer performance of this pattern is evaluated by a numerical method, and the results are compared with those obtained, under identical mass flow rate, when the VGs are mounted only on one surface of the fin. The results reveal that using this new pattern the height of VGs can be reduced and still obtain satisfactory heat transfer enhancement, while the pressure drop is reduced. The results also reveal that if VGs on one surface of the fin is determined, the locations where VGs are mounted on other surface of the same fin are very important, with configurations studied in this paper, depending on the value of Reynolds number, there exists an optimum location with which best heat transfer performance can be obtained.     

Determination of heat transfer correlations for plate-fin-and-tube heat exchangers

This paper presents a numerical method for determining heat transfer coefficients in cross-flow heat exchangers with extended heat exchange surfaces. Coefficients in the correlations defining heat transfer on the liquid- and air-side were determined using a non-linear regression method. Correlation coefficients were determined from the condition that the sum of squared fluid temperature differences at the heat exchanger outlet, obtained by measurements and those calculated, achieved minimum. Minimum of the sum of the squares was found using the Levenberg-Marquardt method. The outlet temperature of the fluid leaving the heat exchanger was calculated using the mathematical model describing the heat transfer in the heat exchanger. Since the conditions at the liquid-side and those at the air-side are identified simultaneously, the derived correlations are valid in a wide range of flow rate changes of the air and liquid. This is especially important for partial loads of the exchanger, when the heat transfer rate is lower than the nominal load. The correlation for the average heat transfer coefficient on the air-side based on the experimental data was compared with the correlation obtained from numerical simulation of 3D fluid and heat flow, performed by means of the commercially available CFD code. The numerical predictions are in good agreement with the experimental data.     

燃气射流冲击传热特性的数值模拟

针对射流传热问题,利用基于RNGk-ε湍流模型的数值方法模拟了射流垂直冲击平板的流动过程,并与实验数据比较,验证了模型的可行性。在此基础上,以火箭喷管入口参数为入口条件,建立了超音速燃气射流垂直冲击平板和冲击浸没平板的计算模型,分析了不同冲击条件下努塞尔数分布规律和温度分布规律, 论述了超音速射流传热的特性及影响传热特性的因素。得到了冲击距离为(14~18)D的努塞尔数取值范围,并表明冲击距离和射流温度是影响传热效率的关键因素;冲击距离增加,传热效率降低,冲击平板表面的射流温度越高,传热效率越高。     

Conjugate conduction-natural convection in an enclosure with time-periodic sidewall temperature and inclination

The unsteady conjugate conduction-natural convection in enclosure is of great theoretical significance and is widely encountered in engineering applications in the areas of fluid dynamics and heat transfer. However, there are relatively few efforts to investigate the unsteady flow physics and heat transfer characteristics in the inclined enclosure of finite thickness walls. In the present work, this problem is numerically investigated by a high accuracy multidomain temporal-spatial pseudospectral method. The enclosure is filled with Boussinesq fluid and is bounded by four finite thickness and conductive walls; one of the vertical sidewall is exposed to time-periodic temperature environment while the opposite sidewall holds constant temperature; the top and bottom walls are assumed to be adiabatic. Particular efforts are focused on the effects of three types of influential factors: the wall thermophysical properties, the time-periodic temperature patterns and the inclination, and the time-periodic flow patterns and heat transfer characteristics are presented. Numerical results reveal that within the present parameter range, the heat transfer rate increases almost linearly with the thermal conductivity ratio and thermal diffusivity ratio but decreases with the inclination angle. Moreover, the heat transfer could be enhanced or weakened by selecting different temperature pulsating period in the case of finite thickness wall, while it is always enhanced if the walls are zero thickness. The back heat transfer and heat transfer resonance phenomena are observed, and their relationships with the time-periodic flow patterns and temperature distributions are analyzed. The findings are helpful to the understandings of the fluid flow and heat transfer mechanisms in the related enclosure configurations, and may be of engineering use in thermal design improvement.     

Pulsating regime corresponding to an obstacle in a stationary inhomogeneous flow

The pulsating regime produced by the presence of a cylindrical cavity in a stationary inhomogeneous supersonic flow is simulated mathematically. The system of equations for an inviscid thermally nonconducting gas is solved by a numerical method based on a two-step difference scheme of second order of approximation. This method makes it possible to calculate in each time step the complete flow field at once, which makes it possible to follow the development of the nonstationary flow, which in the present case is a pulsating flow. The flow pattern in the pulsating regime is studied in detail. The pressure pulsations in the cavity are due to the alternating passage through it of shock waves and rarefaction waves, and the pulsations are nonlinear. The influence of the basic parameters on the characteristics of the pulsating flow is studied and some estimates are made.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 64–71, September–October, 1979.     

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