Heat Transfer In Oscillating Flow Research Articles

An experimental investigation on parallel-plate finned heat exchanger working with cryogenic oscillating helium flow

The space-cycle averaged Nusselt number is used to characterize the heat transfer characteristics of cold end heat exchanger in cryocoolers working with cryogenic oscillating helium flow. An experimental setup for cryogenic oscillating flow heat transfer measurement was designed and established. According to the experimental data obtained, it is evident that low temperature is significantly disadvantageous for heat transfer in oscillating flow. Similar to the experimental results at room temperature, under different mass flow rates, the heat exchanger has its own corresponding optimal Reynolds number at certain temperature for maximized heat transfer capacity.

Heat transfer characteristics of the oscillating flows of different working gases in U-shaped tubes of a Stirling engine

Oscillating flow heat transfer characteristics are quite different from those of steady flows, which make it difficult to predict the effective input heating power of a Stirling engine with existing empirical formulas. This paper reports the experimental study of oscillating flow heat transfer features of helium, nitrogen and carbon dioxide in U-shaped tubes when the pressure, revolving speed and heating temperature are in the ranges of 0.2 MPa–0.9 MPa, 150 r/min–380 r/min and 332 °C–516 °C, respectively. The heat transfer coefficient increases with the increase of the pressure or revolving speed, and it declines when the heating temperature increases. The influences of different working media are discussed. Helium has the highest heat transfer coefficient of 115.2–192.1 W/(m2·K) and the lowest temperature difference of 30.1 °C–54.0 °C between tube walls and the working medium. The heat transfer coefficients of carbon dioxide and nitrogen are slightly worse than those of helium, and the temperature differences are 45.2–80.7 °C and 41.0–70.1 °C, respectively. Heat transfer formulas were proposed, considering the working medium, pressure, revolving speed and heating temperature; the error of the formulas are within 5.2–11.1% when the Reynolds number ranges from 61.7 to 3184.6.

CFD study of heat transfer for oscillating flow in helically coiled tube heat-exchanger

The heat transfer and pressure drop for oscillating flow in helically coiled tube heat-exchanger were numerically investigated based on the Navier–Stokes equations. The correlation of the average Nussel number and average friction factor were proposed considering the frequency and the inlet velocity. The oscillating flow heat transfer problems are influenced by many factors. Hence we need an easy way to reduce the numbers of simulation or experiment. Therefore, the method of uniform design was adopted and the feasibility of this method was verified. The field synergy principle was used to explain the heat transfer enhancement of oscillating flow in helically coiled tube heat-exchanger. The result shows that the smaller the volume average field synergy angle in the helically coiled tube, the better the rate of heat transfer.

A one-dimensional heat transfer model for parallel-plate thermoacoustic heat exchangers

A one-dimensional (1D) laminar oscillating flow heat transfer model is derived and applied to parallel-plate thermoacoustic heat exchangers. The model can be used to estimate the heat transfer from the solid wall to the acoustic medium, which is required for the heat input/output of thermoacoustic systems. The model is implementable in existing (quasi-)1D thermoacoustic codes, such as DeltaEC. Examples of generated results show good agreement with literature results. The model allows for arbitrary wave phasing; however, it is shown that the wave phasing does not significantly influence the heat transfer.

Influence of compression-expansion effect on oscillating-flow heat transfer in a finned heat exchanger

Compression and expansion of a working gas due to the pressure oscillation of an oscillating flow can lead to a temperature variation of the working gas, which will affect the heat transfer in the oscillating flow. This study focuses on the impact of the compression-expansion effect, indicated by the pressure ratio, on the heat transfer in a finned heat exchanger under practical operating conditions of the ambient-temperature heat exchangers in Stirling-type pulse tube refrigerators. The experimental results summarized as the Nusselt number are presented for analysis. An increase in the pressure ratio can result in a marked rise in the Nusselt number, which indicates that the compression-expansion effect should be considered in characterizing the heat transfer of the oscillating flow, especially in the cases with a higher Valensi number and a lower maximum Reynolds number.

Analytical investigation of oscillating flow heat transfer in pulse tubes

In the present paper, a two-dimensional compressible oscillating flow in the tube section of a pulse tube refrigerator system is modeled, based on the successive approximation method. In this respect, the variables are expanded up to second-order terms. For pulse tubes with an outer adiabatic surface, the conservation equations of mass, momentum, energy, and the equation of state for the ideal gas are applied. The effects of operating frequency and taper angle on the temperature distribution, heat transfer behavior, and time-averaged enthalpy flow during a cycle are investigated. Increasing the frequency leads to a higher heat transfer rate in the pulse tube. The enthalpy flow, as the cooling performance representative of the pulse tube, reaches maximum for an optimum convergent taper angle. The temperature distribution and heat transfer process along the axial direction, as well as the phase behavior of the heat transfer coefficient, are also studied. It is shown that, moving from the cold to the hot end of the pulse tube, the temperature variation domain and heat transfer rate decrease.

Oscillating flow in a stirling engine heat exchanger

Three heat exchangers exist in modern Stirling engines: a heater, a cooler, and a regenerator. Here a study that deals principally with tubular heaters and coolers is carried out. The calculation procedure for the oscillating flow heat transfer is presented. Literature sources are studied to find the most suitable correlations by comparing them to each other and to the classical turbulent flow correlations encountered in the literature. The enhancement of heat transfer by means of a few circumferential slots inside the tubes and the pressure losses of oscillatory flow are discussed. Non-circular cross-section conduits with rectangular and triangular cross-sections are investigated and compared to the smooth circular tubes. The increment of the performance of an idealised Stirling engine with slotted heat exchanger tubes is compared to the case with smooth ones. The ratio of the gain in the shaft power and pumping losses is 2.22. The Carnot efficiency increment is 2.7%.

Estimation of heat transfer in oscillating annular flow using artifical neural networks

In this study, the prediction of heat transfer from a surface having constant heat flux subjected to oscillating annular flow is investigated using artificial neural networks (ANNs). An experimental study is carried out to estimate the heat transfer characteristics as a function of some input parameters, namely frequency, amplitude, heat flux and filling heights. In the experiments, a piston cylinder mechanism is used to generate an oscillating flow in a liquid column at certain frequency and amplitude. The cycle-averaged values are considered in the calculation of heat transfer using the control volume approach. An experimentally evaluated data set is prepared to be processed with the use of neural networks. Back propagation algorithm, the most common learning method for ANNs, is used for training and testing the network. Results of the experiments and the ANN are in close agreements with errors less than 5%. The study showed that the ANNs could be used effectively for modeling oscillating flow heat transfer in a vertical annular duct.

Characterizing heat transfer coefficients for heat exchangers in standing wave thermoacoustic coolers

The thermal performance of heat exchangers within a thermoacoustic cooler was investigated. Experimental procedures and calculation methods to evaluate the oscillating flow heat transfer coefficients were developed. Dimensionless heat transfer coefficients, or Colburn-j factors, were estimated based on the oscillating-flow variables and compared with results from steady-flow measurements. The results were also compared with heat transfer coefficients predicted from a boundary-layer conduction model, and methods that utilize steady-flow correlations with Reynolds numbers that characterize oscillating flow conditions. Although the boundary layer model is commonly employed for thermoacoustic calculations, it did not accurately predict heat transfer coefficients and the influence of Reynolds number on heat transfer performance. However, accurate predictions were obtained using a steady-flow correlation and a modified Reynolds number that accounts for the oscillating flow field, assuming a half-cycle Reynolds number correction value of 0.109.

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.