Heat Transfer Contours Research Articles

Augmented heat transfer in tubular heat exchanger fitted with V-baffled tapes

Effects of insertion of a straight tape mounted with double-sided V-baffles used as a longitudinal vortex generator (LVG) on turbulent convection heat transfer behaviors in a constant heat-fluxed tube in the Reynolds number (Re) range of 4120 to 25,800 are experimentally and numerically investigated. In the present experiment, the V-baffles at a fixed attack angle (α = 30°) were placed on the tape with two arrangements: V-apex pointing downstream and upstream (hereinafter called “V-down” and “V-up”). The V-down and up tapes with three baffle height ratios, (RB = b/D = 0.20, 0.15, 0.10) and pitch ratios, (RP = P/D = 1.5, 1.0, 0.5) were tested. Data of the current smooth or plain tube were also examined for comparison. The experimental results have shown that the employ of the V-down tape generally yields higher heat transfer, friction loss and thermal performance than that of the V-up one. A new derived thermal-performance enhancement factor (TEF) is proposed. The maximum TEF of the V-down tape is around 2.07 at RB = 0.15, RP = 1.0 and is, in average, about 1.4% higher than the V-up one having its peak of 2.04. A 3D numerical V-baffled-tube flow model was also simulated to examine the fluid flow and heat transfer patterns. The numerical validation was seen in close agreement with available experimental data. Numerical heat transfer contours and flow structures were also reported.

Effect of nonuniform pin size on heat transfer in a rotating rectangular channel with pin-fin arrays

Internal channels to cool the trailing edges of gas turbine blades were typically installed with pin-fin arrays for higher heat transfer enhancement. In this study, the heat transfer characteristics in a rotating rectangular channel (aspect ratio = 4:1) with differently sized pin-fin arrays were experimentally investigated. Liquid crystal thermography was used to measure heat transfer on the endwall in a staggered pin-fin array at a channel orientation of 135°. The staggered array was composed of four or three pins in a row. Pins of varying diameters were inserted in every row exhibiting four pins; either two or four pins were replaced. The rotation numbers ranged from 0 to 0.33, and the Reynolds numbers ranged from 10,000 to 20,000. The results revealed that flow acceleration, separation, and wake flow contributed to various heat transfer contours. Larger pins contributed to higher local flow velocities through fin spacing and produced higher heat transfer distributions. The effect of rotation improved heat transfer enhancement on both trailing and leading surfaces. The friction factor ratio increased when more large-sized pins were installed in the array.

ОПРЕДЕЛЕНИЕ КОЭФФИЦИЕНТА КОНВЕКТИВНОГО ТЕПЛОПЕРЕНОСА В ГИДРОАККУМУЛЯТОРЕ С ТЕПЛОВЫМ РЕГУЛИРОВАНИЕМ ДЛЯ УСЛОВИЙ НЕВЕСОМОСТИ

In the modern world with the increasing power of spacecraft heat dissipation, the need has arisen to use efficient heat removal systems with two-phase heat transfer contours. Their advantages are determined by the fact that they can carry a much larger amount of heat per unit of consumption than when using a single-phase coolant. The energy consumption of the pump for pumping the coolant is insignificant, and the use of heat exchange during boiling allows you to maintain the temperature of objects almost the entire length of the circuit close to the saturation temperature. All heat transfer processes occurring with a change in the state of aggregation of a substance (boiling, condensation) occur much more intensively than during convective heat exchange in a single-phase liquid. A feature of this system is the change in mass of the coolant in the circuit when changing the operating modes of the two-phase heat transfer system. To regulate the amount of coolant in the circuit, as well as to maintain a predetermined pressure (boiling point), a hydraulic accumulator with thermal regulation (HCA) is used. The actual processes of heat and mass transfer in HCA are non-equilibrium, which complicates the calculation of the thermal control system. This paper describes the concept of a nonequilibrium mathematical model for calculating heat and mass transfer processes in HCA. It is shown that the nonequilibrium of processes can be taken into account by the convective heat transfer coefficient “k” in the mathematical model of HCA: k = 1 corresponds to the absence of convection; k > 100 corresponds to an equilibrium process. Based on the analysis of the flight space experiment for heating HCA, a prediction was made of the value of the convective heat transfer coefficient k under zero-gravity conditions. To assess the influence of the “k” value on the process of regulating the two-phase heat transfer contour under weightless conditions, it is recommended to use the values k = 30 ± 15. In ground-based experiments at high-intensity convection in HCA, processes are much more equilibrium than in zero gravity. It is concluded that the equilibrium process (high values of k) can be considered as more conservative concerning the regulatory process in weightlessness.

Heat Transfer and Friction Measurement in Pin-Fin Arrays Under Mist Flow Condition

Abstract Effects of air/water mist flow on endwall heat transfer in a square channel were experimentally investigated using infrared thermography. The purpose was to study the detailed heat transfer contour variation caused by the generation of the liquid films. The surface was roughened with staggered partial pin-fin arrays to enhance flow mixing and liquid entrainment. Two streamwise spacings (Xp/d = 3 and 6) of the fin array were investigated. The gas Reynolds number ranged from 7900 to 24,000. The calculated droplet deposition velocity was comparable to the literature results and was not substantially affected by the gas Reynolds number or fin spacing. For the pin-fin array, heat transfer was dominated by the water accumulation and liquid film formation, which was dependent on the carrier gas flow rate and fin spacing. Furthermore, thick liquid fragments entrained between the fins substantially enhanced local convective heat transfer coefficients. The average heat transfer enhancement on the finned surfaces using mist flow was four times as high as the air flow. The pressure drop from the mist flow was two times as high as the air flow.

Endwall heat transfer and fluid flow around an inclined short cylinder

Measurements of endwall heat transfer and flow field around a short single cylinder have been performed to examine the influence of cylinder inclination at low Reynolds number ( Re D = 1.0 × 10 4). Both ends of the cylinder are attached to the endwalls and the length-to-diameter ratio of the cylinder varies from 2.7 to 4, depending on the inclination angle. Endwall heat transfer contours (obtained from transient liquid crystals) and endwall flow visualization results consistently indicate that the interaction between the horseshoe vortices around the cylinder and the wakes shed from the cylinder varies with the inclination. Spanwise pressure gradient induced by the inclination causes: (i) skewing of the upstream main flow as it approaches the cylinder; (ii) formation of a jet-like flow immediately downstream of the cylinder, followed by its impingement onto the endwall; and (iii) skewed separation line along the cylinder span from the cylinder axis.

Study on flow and heat transfer of multiple impingement jets

Multiple jets are often used to obtain high heat transfer in a wide area. Heat and flow behavior of multiple jets show more complicated characteristics due to the existence of the interactions between adjoining jets, and between jets and spent flows. To clarify the influence of the flow behavior on heat transfer fields, the time and spatial heat transfer coefficients over the impingement plate were measured using an infrared radiometer when four jets impinged to the impingement plate for various jets arrangements and separating distances from jet exit to the impingement plate. The flow fields were also visualized by means of a water table. The heat transfer characteristics are made clear from the results of the instantaneous heat transfer contour or temperature fluctuation values. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 419–431, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20073

Study on Flow and Heat Transfer of Multiple Impingement Jets

Multiple jets are often used to obtain high heat transfer in wide area. Heat and flow behaviour of multiple jets show more complicate characteristics due to the existence of the interactions between adjoining jets, and between jets and spent flows. To clarify the influence of the flow behavior on heat transfer fields, the time and spatial heat transfer coefficients over the impingement plate were measured using an infrared radiometer when four jets impinged to the impingement plate for various jets arrangement and separating distance from jet exit to the impingment plate. The flow fields were also visualized by means of a water table. The heat transfer characteristics is made clear from the results of the instantaneous heat transfer contour or temperature fluctuation values.

F234 衝突噴流群の伝熱特性

The multiple jets are often used to obtain the high heat transfer in the wide area. The heat and flow of multiple jets show more complicate characteristics due to the existence of the interactions between adjoining jets, and between jets and spent flows. To clarify the influence of the flow behavior on heat transfer fields, the time and spatial heat transfer profiles over the impingement plate were measured using an infrared radiometer when four jets impinge to the impingement plate for various jets arrangement and separating distance from jet exit to the impingement plate. The flow fields were also visualized with water table. The heat transfer characteristics make a clear from the results of the instantaneous heat transfer contour or temperature fluctuation values.

Visualisation of turbulent spots under the influence of adverse pressure gradients

In this paper, a method of using temperature sensitive liquid crystal to visualize the thermal footprints of turbulent spots is described. It is employed to study the nature and development of artificially created turbulent spots in a laminar boundary layer over a heated surface under the influence of adverse pressure gradients in a water flow experiment. This method not only provides qualitative information about the structures and growth of the spot but also contours of heat flux beneath an entire spot as it propagates downstream. Furthermore, the lateral spreading angle and propagation rate of turbulent spots are obtained by analysing the colour images and heat transfer contours. The results are compared with those obtained by other researchers using different experimental methods.

Heat Transfer Measurements to a Gas Turbine Cooling Passage With Inclined Ribs

The local heat transfer coefficient distribution over all four walls of a large-scale model of a gas turbine cooling passage have been measured in great detail. A new method of determine the heat transfer coefficient to the rib surface has been developed and the contribution of the rib, at 5 percent blockage, to the overall roughened heat transfer coefficient was found to be considerable. The vortex-dominated flow field was interpreted from the detailed form of the measured local heat transfer contours. Computational Fluid Dynamics calculations support this model of the flow and yield friction factors that agree with measured values. Advances in the heat transfer measuring technique and data analysis procedure that confirm the accuracy of the transient method are described in full.