Heat transfer and flow in spiral channels and coils

An analysis of the experimental results for heat transfer on convex and concave walls of slotted spiral channels as well as for local heat transfer around the periphery and along the tube of the coils is presented. Great attention is paid to examining surface friction and its pulsations on the hydrodynamic entrance section and under stabilized coil flow conditions. The transition from laminar to turbulent flow on the convex and concave walls of slotted spiral channels and around the periphery of the tube of different-curvature coils is analyzed.Lithuanian Power Engineering Institute. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 6, pp. 670–680, June, 1993.     

室外换热器分液装置的实验研究

制冷剂分配不均现象是室外换热器研究的重点内容.本文设计了两种隔板形式,共制作7种样件放置于换热器集管中进行实验研究.通过实验分析了换热器用作蒸发器的6种工况与用作冷凝器的4种工况下的制冷剂分配情况及换热性能.结果表明:本文利用挡流板实现了良好的制冷剂分配效果,两种挡流隔板(A和B)的加入均有助于改善换热器制冷剂分配不均...     

Experimental investigation of ice slurry heat transfer in horizontal tube

Heat transfer of ice slurry flow based on ethanol–water mixture in a circular horizontal tube has been experimentally investigated. The secondary fluid was prepared by mixing ethanol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature -4.4 °C). The heat transfer tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 22% depending on test performed. Measured heat transfer coefficients of ice slurry are found to be higher than those for single phase fluid, especially for laminar flow conditions and high ice mass fractions where the heat transfer is increased with a factor 2 in comparison to the single phase flow. In addition, experimentally determined heat transfer coefficients of ice slurry flow were compared to the analytical results, based on the correlation by Sieder and Tate for laminar single phase regime, by Dittus–Boelter for turbulent single phase regime and empirical correlation by Christensen and Kauffeld derived for laminar/turbulent ice slurry flow in circular horizontal tubes. It was found that the classical correlation proposed by Sieder and Tate for laminar forced convection in smooth straight circular ducts cannot be used for heat transfer prediction of ice slurry flow since it strongly underestimates measured values, while, for the turbulent flow regime the simple Dittus–Boelter relation predicts the heat transfer coefficient of ice slurry flow with high accuracy but only up to an ice mass fraction of 10% and Re_cf > 2300 regardless of imposed heat flux. For higher ice mass fractions and regardless of the flow regime, the correlation proposed by Christensen and Kauffeld gives good agreement with experimental results.     

Measurement of the heat transfer characteristics of gas flow in fine channel heat exchangers used for microminiature refrigerators

The heat transfer characteristics have been measured for the flow of gas in the fine channels of the heat exchangers used for microminiature J-T refrigerators. The test channels were fabricated using a photolithographic technique similar to that used for the fabrication of the refrigerators. The unique features of such channels are their asymmetric roughness, their large relative roughness and a large variation of the heat flux and temperature over the walls of the channels. The tests involved both laminar and turbulent flow regimes. The results we have obtained are useful for designing heat exchangers in microminiature refrigerators.     

Forced convection of low temperature nitrogen gas in rectangular channels with small aspect ratio

Forced convection of low temperature (80-150 K) nitrogen gas flowing through rectangular channels with hydraulic diameters of 0.513-1.814 mm and aspect ratios of 0.013-0.048 has been investigated experimentally. Close attention was focused on the effects of channel depth and heat addition on the heat transfer and flow characteristics, the transition from laminar to turbulent flow and the existence of an optimum channel depth. A dimensionless heating number was adopted to characterize the heating effect. The experimental correlation developed for the Nusselt number shows that the heat addition is the most important effect, followed by the channel aspect ratio, Reynolds number and Prandtl number.     

A Novel Approach for Magnet Leads

An integrated approach to magnet lead design is presented which incorporates innovations and design enhancements over a conventional design. The primary innovation is that a set of baffles is used to transfer heat from the leads to the Helium gas, as opposed to direct thermal contact between the two. Based on this design approach, we have established a calculational method to compute boiloff rates and temperature profiles. We have investigated the feasibility of such a baffle cooled approach by conducting boiloff calculations for various lead geometries. Leads with graded cross sectional areas, which are readily incorporated into such a system, significantly lower the boiloff rate compared with uniform area leads.     

Assessment and analysis of binary hybrid nanofluid impact on new configurations for curved-corrugated channel

The current numerical paper introduces the flow and heat transfer characteristics across a new configuration channel, namely: the curved-corrugated channel, using binary hybrid nanofluid. E-shaped baffles with different geometrical parameters have been employed while CuO / MgO-water nanofluid is experimentally prepared with different volume fractions 0.0–5%. Measured thermophysical properties is utilized to simulate the flow and heat transfer characteristics by adopting the κ-ε model. The influences of corrugations, baffles, and geometric parameters; gap ratio (GR = 0.2,0.3,0.4, and 0.5), blockage ratio (BR = 0.2,0.25,0.3, and 0.35), and pitch angle (β = 10°, 12.5°, and 15°) at different Reynolds number (8000–28000) are evaluated using thermal–hydraulic performance method. The outcomes show that vortex flow and increased turbulence will increase heat transfer due to influences of corrugations and baffles. It is confirmed that the flow variations governed by the geometric parameters of the design and the best performance produce at lowest pitch angle 10°, lowest gap ratio (GR = 0.2) and highest blocking ratio (BR = 0.35). Regards the fluid medium, CuO / MgO particles improve the thermophysical properties of the base fluid and thereby boost the thermal performance of the system. It has found new correlations between the Nusselt number, friction Factor and design parameters of tested channel with using binary hybrid nanofluid.     

Experiments with micro-fin tube in single phase

This work shows heat transfer and friction characteristics for water single-phase flow in micro-fin tubes. The analysis of thermal and hydraulic behavior from a laminar to a turbulent flow was carried out in an experimental setup with a 9.52 mm diameter micro-fin tube. The tube was wrapped up with an electrical resistance tape to supply a constant heat flux to its surface. Different operational conditions were considered in the heating tests. The inlet and outlet temperatures, differential wall temperatures along the tube, pressure drop and flow rate were measured. The relationships of heat flux and flow rate with heat transfer coefficient and pressure drop were analyzed. Under the same conditions, comparative experiments with an internally smooth tube were conducted. The micro-fin tube provides higher heat transfer performance than the smooth tube (in turbulent flow h_micro-fin/h_smooth=2.9). In spite of the increase in pressure drop (Δp_micro-fin/Δp_smooth=1.7) the heat transfer results were significantly higher (about 80%). This shows the advantages of this enhanced configuration in thermal performance related to conventional tubes. The smooth tube results were validated by the comparison with the Dittus–Boelter and Gnielinski correlations. For the micro-fin tube an empirical correlation to the heat transfer coefficient adjusted from the set of measured data is proposed. The values obtained are in conformity with experimental results.     

Hydrodynamics and heat transfer in turbulent pipe flow of liquid under conditions of monotonic time variation of the flow rate

The available experimental and theoretical studies of hydrodynamics and heat transfer in turbulent pipe flow of liquid under conditions of monotonic increase or decrease of the flow rate are reviewed and analyzed. The reason is found why the effect of hydrodynamic nonstationarity on heat transfer and skin friction coefficient turns out to be different from that in the case of laminar flow. The differences in this effect on heat transfer and drag are treated. The experimentally observed effects are reproduced most accurately when simulated on the basis of equations for turbulent stresses and heat fluxes.__________Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 2, 2005, pp. 212–222.Original Russian Text Copyright © 2005 by E. P. Valueva.     

Convective heat transfer coefficients for near-horizontal two-phase flow of nitrogen and hydrogen at low mass and heat flux

Correlations for convective heat transfer coefficients are reported for two-phase flow of nitrogen and hydrogen under low mass and heat flux conditions. The range of flowrates, heat flux and tube diameter are representative of thermodynamic vent systems (TVSs) planned for propellant tank pressure control in spacecraft operating over long durations in microgravity environments. Experiments were conducted in normal gravity with a 1.5° upflow configuration. The Nusselt number exhibits peak values near transition from laminar to turbulent flow based on the vapor Reynolds number. This transition closely coincides with a flow pattern transition from plug to slug flow. The Nusselt number was correlated using components of the Martinelli parameter and a liquid-only Froude number. Separate correlating equations were fitted to the laminar liquid/laminar vapor and laminar liquid/turbulent vapor flow data. The correlations give root-mean-squared (rms) prediction errors within 15%.     

Application of CNT nanofluids in a turbulent flow heat exchanger

Nanofluids have received much attention since its discovery owing to its enhanced thermal conductivity and heat transfer characteristics which makes them a promising coolant in heat transfer application. In this study, the enhancement in heat transfer of carbon nanotube (CNT) nanofluids under turbulent flow conditions is investigated experimentally. The CNT concentration was varied from 0.051 to 0.085 wt%, respectively. The nanofluid suspension was stabilised by gum arabic through a process of homogenisation and water bath sonication at 25 °C. The flow rate of cold fluid (water) is varied from 1.7 to 3 L/min, while flow rate of the hot fluid is varied between 2 and 3.5 L/min. Thermal conductivity, density, and viscosity of the nanofluids are also measured as a function of temperature and CNT concentration. The experimental results were validated with theoretical correlations for turbulent flow available in the literature. Results showed an enhancement in heat transfer between 9% and 67% as a function of temperature and CNT concentration.     

Modern Approaches for Calculating Flow Parameters during a Laminar–Turbulent Transition in a Boundary Layer

We analyze modern methods for calculating heat and hydrodynamic flow parameters in a boundary layer during the laminar–turbulent transition. The main approaches for describing the phenomenon of laminar–turbulent transition are examined. Each approach is analyzed. The manner in which different factors influence the laminar–turbulent transition is studied. An engineering model of the laminar–turbulent transition in a high-velocity flow is presented.     

Heat transfer under conditions of turbulent pipe flow of electrically conducting liquid in longitudinal magnetic field in the region of hydrodynamic stabilization

Numerical simulation is performed of heat transfer under conditions of turbulent pipe flow in the vicinity of entry to longitudinal magnetic field. Use is made of the model of turbulence which was previously employed for performing calculations in the region of stabilized flow and heat transfer. The model describes the suppression of turbulence by the magnetic field and the laminarization of turbulent (at the pipe inlet) flow. The calculation results agree well with the experimental data on heat transfer and temperature profiles in the initial thermal region. The effect made on heat transfer by Joule heat release from electric currents caused by turbulent fluctuations is investigated.     

Boundary layer on a permeable wall with suction of gas

The differential model of turbulence, supplemented with transport equation for turbulent heat flux, is used to perform a numerical investigation of the boundary layer on a permeable wall with suction of gas. It is demonstrated that the protraction of transition from laminar to turbulent mode of flow and the laminarization of the initial turbulent boundary layer occur under conditions of suction of gas. This is evidenced both by the behavior of integral and local characteristics of flow and heat transfer and by the degeneracy of turbulence when the suction of laminar turbulent layer becomes asymptotic. The critical values of the suction parameter are determined.     

Heat transfer and hydrodynamics in cooling channels of superconducting devices

The results of experimental studies of heat transfer, critical heat flux and pressure drop in forced flow of cryogens are presented. Analysis of different flow and heat transfer conditions was carried out using the results of present studies and the data of other authors. Significant effects of operating conditions on hydraulic and temperature characteristics of cooling channels in superconducting devices were demonstrated. The influence of a heating transient on heat transfer to liquid helium was examined. Correlations were developed which are in close agreement with available heat transfer and pressure drop data for forced flow of cryogens.     

The Effect of Hydrodynamic Unsteadiness on the Flow Structure and on the Coefficients of Heat Transfer and Skin Friction under Conditions of Turbulent Pipe Flow of Heat-Transfer Agent

Critical analysis is made of the current status of investigations of heat transfer and skin friction under conditions of unsteady-state turbulent pipe flow. This analysis is based on the experimental data on the structure of turbulent flows and heat transfer. Comparison is made of experimental and theoretical results.     

Experimental investigation on TBAB clathrate hydrate slurry flows in a horizontal tube: Forced convective heat transfer behaviors

Tetra-n-butyl-ammonium bromide (TBAB) clathrate hydrate slurry (CHS) is one kind of secondary refrigerants, which is promising to be applied into air-conditioning or latent-heat transportation systems as a thermal storage or cold carrying medium for energy saving. It is a solid–liquid two phase mixture which is easy to produce and has high latent heat and good fluidity. In this paper, the heat transfer characteristics of TBAB slurry were investigated in a horizontal stainless steel tube under different solid mass fractions and flow velocities with constant heat flux. One velocity region of weakened heat transfer was found. Moreover, TBAB CHS was treated as a kind of Bingham fluids, and the influences of the solid particles, flow velocity and types of flow on the forced convective heat transfer coefficients of TBAB CHS were investigated. At last, criterial correlations of Nusselt number for laminar and turbulent flows in the form of power function were summarized, and the error with experimental results was within ±20%.     

Simulations of flow through fluid/porous layers by a characteristic‐based method on unstructured grids

An upwind characteristic‐based finite volume method on unstructured grids is employed for numerical simulation of incompressible laminar flow and forced convection heat transfer in 2D channels containing simultaneously fluid layers and fluid‐saturated porous layers. Hydrodynamic and heat transfer results are reported for two configurations: the first one is a backward‐facing step channel with a porous block inserted behind the step, and the second one is a partially porous channel with discrete heat sources on the bottom wall. The effects of Darcy numbers on heat transfer augmentation and pressure loss were investigated for low Reynolds laminar flows. The results demonstrate the accuracy and robustness of the numerical scheme proposed, and suggest that partially porous insertion in a channel can significantly improve heat transfer performance with affordable pressure loss. Copyright © 2001 John Wiley & Sons, Ltd.     

Hydrodynamics and heat transfer in curvilinear channels of rectangular cross section

Experimental results of a study of hydraulic resistance and heat transfer in helical channels of rectangular cross section for laminar and turbulent flows in the isothermal and nonisothermal regimes are given.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 34, No. 6, pp. 994–1000, June, 1978.