Heat Transfer In Oscillatory Flow Research Articles

Study of heat transfer in oscillatory flow for a Stirling engine heating tube inserted with spiral spring

The heater of Stirling engine transfers heat from external heat sources to working gas. Improving its heat transfer performance is favourable to save external heat sources and decrease dead volume of engine. In this study, the flow and heat transfer characteristics of reciprocating oscillatory flow in a tube inserted with spiral spring in a Stirling engine heater were investigated. The results indicate that the Nusselt number (Nu), friction factor (f) and performance evaluation criterion (PEC) increased with spiral height (h) and decreased with spiral pitch (p) for enhanced tube within the range of parameters in this study. For the optimal enhanced tube, the PEC was equal to 1.22 and the outlet temperature was significantly improved compared to smooth tube. The maximum improvement was 115 K in entry process and 94 K in return process, and the average improvement was 64 K in entry process and 34 K in return process. The extra pressure consumption in the optimal enhanced tube was less than 0.0013 MPa compared to smooth tube, which could be negligible contrasting to the overall pressure in heating tube (1.5 ∼ 2.5 MPa). Thus, the benefits are apparent for heating tube inserted with spiral spring in Stirling engine.

振動流用熱交換器の性能評価

Heat exchangers are some of the most important parts in thermoacoustic devices. In oscillatory flow condition, the flow and temperature fields around the heat exchangers are quite complex and it may significantly affect their heat transfer behavior. As a result, we cannot directly apply the heat transfer definition for a steady flow to the design of heat exchangers operating in oscillatory flow conditions. Moreover, the fundamental knowledge of heat transfer in oscillatory flow is still not well established. By measuring the heat transfer between the heat exchangers and working fluid in oscillatory flow, we define evaluation index of heat transfer performance. We confirm that incorporating the device improve performance.

Thermal performance of finned-tube thermoacoustic heat exchangers in oscillatory flow conditions

Heat exchangers play a key role in the overall performance of thermoacoustic devices. Due to the complex nature of oscillatory flows, the underlying mechanism of heat transfer in oscillatory flows is still not fully understood. This work investigates the effect of fin length and fin spacing on the thermal performance of finned-tube heat exchangers. The heat transfer rate between two finned-tube heat exchangers arranged side-by-side in an oscillatory flow was measured over a range of testing conditions. The results are presented in terms of heat transfer coefficient and heat transfer effectiveness. Comparisons are made between experimental results of this work and a number of models, such as the Time-Average Steady-Flow Equivalent (TASFE) model, the Root Mean Square Reynolds Number (RMS-Re) model and the boundary layer conduction model, as well as several empirical correlations in literature. A new empirical correlation is proposed to be used for the prediction of thermal performance for finned-tube heat exchangers in oscillatory flows. The uncertainties associated with the measurement of heat flux are estimated.

Development of experimental techniques for measurement of heat transfer rates in heat exchangers in oscillatory flows

Heat exchangers are important components of thermoacoustic devices. In oscillatory flow conditions, the flow and temperature fields around the heat exchangers can be quite complex, and may significantly affect heat transfer behaviour. As a result, one cannot directly apply the heat transfer correlations for steady flows to the design of heat exchangers for oscillatory flows. The fundamental knowledge of heat transfer in oscillatory flows, however, is still not well-established. The aim of the current work is to develop experimental apparatus and measurement techniques for the study of heat transfer in oscillatory flows. The heat transferred between two heat exchangers forming a couple was measured over a range of testing conditions. Three couples of finned-tube heat exchangers with different fin spacing were selected for the experiment. The main parameters considered were fin spacing, fin length, thermal penetration depth and gas displacement amplitude. Their effects on the heat exchanger performance were studied. The results were summarised and analysed in terms of heat transfer rate and dimensionless heat transfer coefficient: Colburn-j factor. In order to obtain the gas side heat transfer coefficient in oscillatory flows, the water side heat transfer coefficient is required. Thus, an experimental apparatus for unidirectional steady test was also developed and a calculation method to evaluate the heat transfer coefficient was demonstrated. The uncertainties associated with the measurement of heat transfer rate were also considered.

G0600-2-3 往復振動流中の熱伝達に関する数値シミュレーション(熱工学部門一般講演(2):伝熱(2))

G0600-2-3 往復振動流中の熱伝達に関する数値シミュレーション(熱工学部門一般講演(2):伝熱(2))

Experimental study of thermoacoustic effects on a single plate Part I: Temperature fields

The thermal interaction between a heated solid plate and the acoustically driven working fluid was investigated by visualizing and quantifying the temperature fields in the neighbourhood of the solid plate. A combination of holographic interferometry and high-speed cinematography was applied in the measurements. A better knowledge of these temperature fields is essential to develop systematic design methodologies for heat exchangers in oscillatory flows. The difference between heat transfer in oscillatory flows with zero mean velocity and steady-state flows is demonstrated in the paper. Instead of heat transfer from a heated solid surface to the colder bulk fluid, the visualized temperature fields indicated that heat was transferred from the working fluid into the stack plate at the edge of the plate. In the experiments, the thermoacoustic effect was visualized through the temperature measurements. A novel evaluation procedure that accounts for the influence of the acoustic pressure variations on the refractive index was applied to accurately reconstruct the high-speed, two-dimensional oscillating temperature distributions.

Longitudinal Heat Transfer in Oscillatory Flows in Pipe Bundles of Various Cross Sections.

The effect of the sectional shape of pipes on the enhancement of longitudinal heat transfer by fluid pulsation in pipe bundles has been investigated. Bundles of circular, triangular, square and hexagonal pipes are considered. Heat transfer characteristics of each pipe and the gross performance as a pipe bundle including the heat conduction in pipe walls, are discussed. The local distribution of longitudinal heat transfer in pipe section reveals the existence of a region where heat is transferred toward the higher temperature end when the pulsation frequency is sufficiently high. As a pipe bundle, only a circular one has narrow gaps among each three pipes, which deteriorate the gross performance of heat transfer. But in the case of the present investigation which assumes acrylic pipes of the same wall thickness, the circular pipe bundle is advantageous to other ones as for the global performance of longitudinal heat transfer including the wall thermal conduction, because the area ratio of pipe walls, whose low thermal conductivity lowers the performance, is least when the pipes are circular.

The Effect of the Pipe Cross-Sectional Shape on the Longitudinal Heat Transfer in Oscillatory Flows in Pipe Bundles.

The enhancement of longitudinal heat transfer by means of fluid pulsation in pipe bundles has been investigated. Bundles of circular, triangular, square and hexagonal pipes are considered. Heat transfer characterisitics of each pipe and the gross performance as pipe bundles including the heat conductance in pipe walls are discussed. The distribution of the longitudinal heat transfer intensity across the pipe section reveals the existance of a region where heat is transferred toward the high temperature end when the pulsation frequency is high. Although the gaps between three circular pipes deteriorate the heat transfer characteristics by fluid in a circular pipe bundle, when acrylic pipes are assumed, the gross performance is the best with a circular pipe bundle, because the area ratio occupied by pipe wall in a cross-section, where the low thermal conductance deteriorates the performance, is least when the pipes are circular.

Heat transfer measurements on a single stack plate

Heat transfer in oscillatory flow with zero mean velocity is different when compared to steady-state flow situations. This is one reason that currently no systematic design methodologies for heat exchangers in such flows exist. For example, instead of the expected heat transfer from a single heated stack plate [Wetzel et al., J. Acoust. Soc. Am. 100 (1996)] to the working fluid, the temperature fields visualized with holographic interferometry indicate that heat is transferred from the working fluid into the stack plate at the edge of the plate. To investigate this behavior the local heat fluxes at the edge of the stack plate were measured. These heat fluxes indicate the expected behavior for a low drive ratio. However, as the drive ratio increases, the resulting heat flux vector at the edge of the plate changes its direction and points into the plate, thus confirming the observations made when analyzing the temperature fields. Through an energy balance the magnitudes of the heat fluxes caused by the thermoacoustic effect were determined. The measured data are in good agreement with numerical and analytical predictions. [Work supported by the Office of Naval Research.]

Limitations of temperature measurements with holographic interferometry in the presence of pressure variations

Abstract In recent years oscillatory flows have shown to be a promising strategy to enhance heat transfer. However, the mechanisms underlying oscillatory heat transfer enhancement are not yet completely understood. One problem, when investigating heat transfer in oscillatory flows experimentally, is to resolve the temperature distribution as a function of time. This is one reason that most studies reported in the literature so far were restricted to frequencies of a few hertz. As shown in this paper, an appropriate tool to investigate oscillatory heat transfer phenomena at higher frequencies (∼1000 Hz) is real time holographic interferometry (HI) combined with high-speed cinematography. In the present paper HI was applied to study acoustically driven flow. To apply HI to such a physical situation it was necessary to expand its applicability to cases where changes in the refractive index are caused not only by temperature changes but also by pressure variations. For this purpose a new evaluation formula that accounts for pressure variations was derived. On the example of the acoustic field, we discuss the impact of the pressure variations on temperature measurements. Additionally, an image processing algorithm was developed that allows the measurement of time dependent temperature distributions. The uncertainties of the temperature measurements introduced by the image processing algorithm were found to be in the range of thermocouple measurements.

Preface

This ninth volume of the serial publication, Annual Review of Heat Transfer, continues the fine tradition of providing improved communication on new, important developments in the general field of heat transfer. As in previous volumes, each chapter in the Annual Review deals with a significant, emerging topic by starting from fundamental physical considerations and then proceeding in a logical fashion to the forefront of recent developments and future challenges. The contributing authors are either young but upcoming, or well-established leaders in these emerging fields. A list of the topics covered in the past two volumes illustrates well this special characteristic of the Annual Review. Volume 7 contained excellent chapters on significant, but wide-ranging heat transfer research and developments in micro- and nano-scale optical and optoelectronic devices, growth of dendritic crystals, polymeric composite processing, active enhancement using electric field, direct-contact transfer with droplets, clean-up of underground toxic wastes, and fluidized beds, while Volume 8 considered such important topics as heat transfer in microelectronics, Rayleigh-Bénard convection, aerodynamics of flame surfaces, radiative transfer in non-isothermal inhomogeneous media, heat pipe analysis and simulation, and high heat flux spray cooling. Continuing with this tradition, the present volume provides informative chapters on bioheat transfer, large scale surface radiative transport, radiative transfer in fibrous media, combustion in porous media, turbine blade coolant passage heat transfer, external flow film boiling, heat transfer in oscillatory flows, and two-dimensional annular flow. The editor is deeply grateful to the participating authors for their excellent contributions, to Begell House for providing invaluable editorial assistance, and to the heat transfer community for its continuing support and input.

NUMERICAL STUDY OF TURBULENT FORCED CONVECTION IN A PERIODICALLY RIBBED CHANNEL WITH OSCILLATORY THROUGHFLOW

Engineering Systems International/Uihon ESI K.K., 2-47-18 Uehara, Shibuya. Tobo 151, Japan SUMMARY Numerical computations are performed on the fully developed flow and heat transfer in a periodically ribbed channel with oscillatory throughflow. A uniform heat flux is imposed at the lower plate of the channel. An externally sustained pressure gradient vanes sinusoidally in time. A low-turbulent-Reynolds-number version of the k+ two-equation model of turbulence is invoked, together with a preferential dissipation modification, to predict the complex turbulent flow field. Computed results indicate that much heat transfer enhancement is expected by increasing the Womersley number, which measures the relative strength of the oscillatory motion to the viscous effects. Turbulent heat transfer in a channel with rib-roughened walls is of much engineering relevance. It is known to be an effective means of heat transfer enhancement in compact heat exchangers, cooling of electronic devices and other types of thermal engineering equipment. A higher heat transfer rate may be expected by imposing on the channel an externally sustained oscillatory pressure gradient, by which better mixing of the medium can be accomplished. It is thought to be a promising technique for cooling high-density electronic circuit boards, for which an excessively high flow rate of the cooling agent is not desirable from a mechanical design point of view. Despite its potential for innovative engineering applications, the available information on turbulent heat transfer in oscillatory flow is limited. Ohmi et d.' measured the velocity profiles of oscillatory througMow in a rectangular duct using hot-wire anemometry. Instantaneous velocity profiles for turbulent oscillatory flow were found to be similar to those of non-oscillatory flow. Consequently, the profiles contained little phase shift, contrasting with their laminar flow counterparts. The same authors have also constructed a flow regime diagram, thereby identifying the role of the mechanically imposed frequency (characterized by the Womersley number) in the flow transition. Their experiments were conducted under an isothermal condition. Features of the flow field and mass transfer rate were investigated for a wavy walled channel by Nishimura et aL2 With a smoothly contoured wall, flow separation and subsequent growth of a vortex, which eventually occupied the entire flow passage, were clearly observable in the acceleration stage. In the deceleration stage accompanying the reversal in the flow direction, these vortices were

A new formulation for unsteady heat transfer of oscillatory flow in a circular tube

Heat Transfer with periodic fluctuation of fluid temperature caused by oscillatory flow or compression expansion can be out of phase with balk fluid-wall temperature difference. Newton's law of convection is inadequate to describe this phenomenon. In order to solve this problem the concept of the complex Nusselt number has been introduced by severla researchers. The complex Nusselt number expresses out of phase excellently while the first harmonic is dominant in the variations of both fluid-wall temperature difference and heat flux. However, in the case of oscillatory flow with non-linear wall temperature distribution, the complex Nusselt number is not appropriate to predict the heat transfer phenomena since the higher order harmonic components appear in periodic temperature variation. Analytic solutions to the heat transfer with an sinusoidal well temperature distribution were obtained to investagate the effect of non-linear wall temperature distribution. A new formula considering the thermal boundary layer was suggested based on the solutions. A comparison was also made with the complex Nusselt number. It was verified that the new formula describes well the heat transfer of oscillating flow even if the first harmonic component is not dominant in the fluid-wall temperature difference.

Longitudinal Heat Transfer in Oscillatory Flows in Pipes With Thermally Permeable Wall

The effect of a thermally permeable wall on the enhanced longitudinal heat transfer by fluid pulsation in a pipe has been investigated. An analytical solution is obtained for the case when heat loss through the wall over a cycle of fluid pulsation is uniform along the pipe. The distribution of time-averaged sectional mean temperature is expressed by a quadratic curve. The rate of longitudinal heat transfer decreases linearly toward the lower temperature end. The amplitude of time-varying sectional mean temperature also decreases toward the same end. The effect of the value of Womersley number on this temperature field is discussed.

Longitudinal Heat Transfer in Oscillatory Flows in Circular Pipes with Heat Loss.

The enhancement of longitudinal heat transfer by means of fluid pulsation in a circular pipe with heat loss through the pipe wall has been investigated. An analytical solution is obtained for the case that heat loss through the pipe wall during one cycle of fluid pulsation is uniform along the pipe. Based on the analytical solution, some characteristics of the phenomenon are discussed. These include the effect of heat loss on longitudinal heat transfer and temperature distributions, as well as the effect of pulsation frequency on the amplitude of temperature pulsation.

Wall Friction and Local Heat Transfer in Oscillatory Flow : Two-dimensional Unsteady Laminar Boundary Layer on a Flat Plate

Wall friction and local heat transfer in an oscillating laminar boundary layer over a flat plate with an unheated starting length are theoretically analyzed. In the analysis, the power series solutions are derived for velocity and temperature distributions in three frequency regions, that is, low frequency (ω*<1), intermediate frequency (1<ω*<1/K), response of a hot film sensor is clarified theoretically. Especially it is shown that the amplitude of heat transfer oscillation with an unheated starting length increase with frequency at low and intermediate regions, and the phase angle is in advance of free stream oscillation at these frequency regions.