Evaporator critical heat flux in the double-wall artery heat pipe

The fact that heat is transferred into a heat pipe through the liquid-saturated evaporator wick gives rise to the so-called boiling limit on the heat pipe capacity. The composite nature of the double-wall artery heat pipe (DWAHP) wick structure makes the prediction of the evaporator superheat (Δ T_crit) and the critical radial heat flux (q_r) very difficult. The effective thermal conductivity of the wick, the effective radius of critical nucleation cavity, and the nucleation superheat, which are important parameters for double-wall wick evaporator heat transfer, have been evaluated based on the available theoretical models. Empirical correlations are used to corroborate the experimental results of the 2 m DWAHP. A heat choke mounted on the evaporator made it possible to measure the evaporator external temperatures, which were not measured in the previous tests. The high values of the measured evaporator wall temperatures are explainable with the assumption of a thin layer of vapor blanket at the inner heating surface. It has been observed that partial saturation of the wick (lean evaporator) causes the capillary limit to drop even though it may be good for efficient convective heat transfer through the wick. The 2 m long copper-water heat pipe had a peak performance of 1850 W at 23 W/cm² with a horizontal orientation.     

Recent achievements in the thermal hydraulics of high heat flux components in fusion reactors

Some components of fusion thermonuclear reactors, such as divertors, plasma limiters, or first-wall armor, are believed to be subjected to operating conditions characterized by extremely high thermal loads. It is therefore necessary to remove from the surface of these components very high heat fluxes, ranging from 2 to 60 MW/m². Water subcooled flow boiling, under conditions of high mass flux, high liquid subcooling, and small to intermediate channel diameter, can accomodate these very high heat fluxes. Further enhancement of the upper limit of cooling, the critical heat flux (CHF), can be obtained by making use of turbulence promoters such as twisted tapes and coiled wires even if coupled with a relevant increase in pressure drop. An overview is presented of recent achievements obtained in water subcooled flow boiling CHF under operating conditions of interest to the thermal hydraulic design of fusion reactors. Observed basic parametric trends—CHF as a function of mass flux, pressure, subcooling, and channel geometry—are outlined, together with findings on the use of CHF enhancement techniques. From experiments it was seen that water subcooled flow boiling allows CHF conditions as high as 228 MW/m² to be achieved under extreme geometric and thermal hydraulic conditions. On the other hand, design and engineering boundary conditions limit variation in these conditions, and a suitable compromise has not yet been reached. Predictive tools are presented for the evaluation of subcooled flow boiling CHF both in straight tubes and with twisted tapes, and are assessed with reference to recent available experimental data.

Although several indications for practical applications can be found in recent achievements, a full understanding of the basic mechanisms of heat transfer and CHF in subcooled flow boiling has not yet been achieved. Future research to overcome the present lack of knowledge in this field is suggested.     

A review of measurements of heat flux density applicable to the field of combustion

The techniques that have been developed for the measurement of heat flux density are reviewed briefly. These techniques may be divided into two broad categories: (1) indirect methods based on the fundamental theories of heat transfer and (2) direct methods using a heat flux density sensor. Various methods are compared in order to stimulate further research and the development of sophisticated techniques for the measurement of heat flux density in the field of combustion.     

The correction of longitudinal heat flux in turbulence measurement

In this paper the influence of temperature on velocity signal in hot-wire measurement of turbulence is analysed. It is pointed out that when the temperature influence is small, the temperature influence on measured intensity of velocity fluctuations is second order small and negligible. However, the temperature influence on measuring longitudinal heat flux is of first order quantity, and must be corrected, or large error will occur. The method to correct the temperature influence on measuring ρ _θu and the procedure to decide experimentally temperature influence coefficient have been given.     

Radiative heat flux characteristics of methane flames in oxy-fuel atmospheres

Oxy-fuel combustion is a promising alternative for power generation with CO₂ capture, where the fuel is burned in an atmosphere enriched with oxygen and CO₂ is used as a diluent. This type of combustion is characterised by uncommon characteristics in terms of thermal heat transfer budget as compared to air supported systems. The study presents experimental results of radiative heat flux along the flame axis and radiant fractions of non-premixed jet methane flames developing in oxy-fuel environments with oxygen concentrations ranging from 35% to 70%, as well as in air. The flames investigated have inlet Reynolds numbers from 468 to 2340. The data collected have highlighted the effects of the flame structure and thermo-chemical properties of oxy-fuel combustion on the heat flux radiated by the flames. It was first observed that peak heat flux increases considerably with oxygen concentration. More generally the radiant fraction increases with both increasing Reynolds number in the laminar regime and oxygen concentration. It was found that despite a difference in flame temperature, the radiative characteristics of the flames (heat flux distributions and radiant fraction) in air were similar to those with 35% O₂ in CO₂. The radiative properties of flames in oxy-fuel atmosphere with CO₂ as diluents appear to be dominated by the flame temperature.     

Experimental investigation of heat transfer and pressure drop characteristics of flow through spirally fluted tubes

Spirally fluted tubes are used extensively in the design of tubular heat exchangers. In previous investigations, results for tubes with flute depths e/D_vi < 0.2 were reported, with most correlations applicable for Re ≥ 5000. This paper presents the results of an experimental investigation of the heat transfer and pressure drop characteristics of spirally fluted tubes with the following tube and flow parameter ranges: flute depth e/D_vi = 0.1−0.4, flute pitch p/D_vi = 0.4−7.3, helix angle θ/90° = 0.3−0.65, Re = 500−80,000, and Pr = 2−7. The heat transfer coefficients inside the fluted tube were obtained from measured values of the overall heat transfer coefficient using a nonlinear regression scheme. The friction factor data obtained consisted of 507 data points. The proposed correlation for the friction factor predicts 96% of the database within ±20%. The heat transfer correlation for the range 500 ≤ Re ≤ 5000 predicts 76% of the database (178 data points) within ±20%, and the correlation for the higher Re range predicts 97% of the 342 data points within ±20%. Comparison of heat transfer and friction data show that these tubes are most effective in the laminar and transition flow regimes. The present results show that the increase of flute depth in the range considered does not improve heat transfer.     

Theoretical analyses on boiling critical heat flux with porous media

Porous media has been widely applied to enhance boiling heat transfer in industry, especially for increasing the value of critical heat flux (CHF). Two cases were considered in the paper: boiling within porous bed and boiling above on porous coatings. For boiling within porous bed, simplified Rayleigh–Taylor stability was analyzed and parametric effects of porous media on boiling critical heat flux were revealed. For boiling above on porous coatings, a simple new critical heat flux model was proposed basing on the analysis of liquid film stability and parametric effect of porous coatings on CHF was elaborated.     

Investigation of saturated critical heat flux in a single, uniformly heated microchannel

A series of tests have been performed to determine the saturated critical heat flux (CHF) in 0.5 and 0.8 mm internal diameter microchannel tubes as a function of refrigerant mass velocity, heated length, saturation temperature and inlet liquid subcooling. The tested refrigerants were R-134a and R-245fa and the heated length of microchannel was varied between 20 and 70 mm. The results show a strong dependence of CHF on mass velocity, heated length and microchannel diameter but no influence of liquid subcooling (2–15 °C) was observed. The experimental results have been compared to the well-known CHF single-channel correlation of Y. Katto and H. Ohno [An improved version of the generalized correlation of critical heat flux for the forced convective boiling in uniformly heated vertical tubes, Int. J. Heat and Mass Transfer 27 (9) (1984) 1641–1648] and the multichannel correlation of W. Qu and I. Mudawar [Measurement and correlation of critical heat flux in two-phase microchannel heat sinks, Int. J. Heat and Mass Transfer 47 (2004) 2045–2059]. The comparison shows that the correlation of Katto–Ohno predicts microchannel data with a mean absolute error of 32.8% with only 41.2% of the data falling within a ±15% error band. The correlation of Qu and Mudawar shows the same trends as the CHF data but significantly overpredicts them. Based on the present experimental data, a new microscale version of the Katto–Ohno correlation for the prediction of CHF during saturated boiling in microchannels has been proposed.     

Prediction of roll temperature with a non-uniform heat flux at tool and workpiece interface

In a metal forming process, plastic deformation of the workpiece takes place at tool and workpiece interface region. Tool has been identified as one of the key parameters in controlling the productivity of any manufacturing industry. The deformation of metals and friction at the contact region produce large amount of heat, a part of that heat is conducted towards the tool where it is removed by forced convection. These cooling and heating cycles finally result in a substantial change in the temperature distribution in the roll. In this paper, an attempt is made to study the temperature and heat flux distribution in the roll by considering a non-uniform heat flux at the roll-workpiece interface for a cold rolling process. Adopting an elemental approach, a methodology has been proposed to model non-uniform heat flux at the interface. For this purpose both tool and workpiece has been considered together, thus a coupled approach is used to model both deformation and heat transfer phenomenon. It is demonstrated that the present approach of modeling is more general than that available in the literature. For example, a constant value of heat flux at the interface that is considered by several investigators is shown to be a special case of the present investigation, particularly when the deformation and relative velocity is very small. It is shown that the error in maximum temperature associated with constant heat flux assumption could be more than 5% in situations when reduction and relative velocity is high. The results are presented for temperature and heat flux distributions in the roll for different operating conditions.a thermal diffusivity, (m²/sec) - B pre-strain coefficient - C yield stress at unit strain, (N/m²) - e rate of deformation heat generation per unit volume, (W/m³) - f friction factor - h heat transfer coefficient, (W/m² °C) - k thermal conductivity, (W/m °C) - K yield stress at unit strain, (N/m²) - L bite length, (m) - n strain hardening exponent - P pressure between tool and workpiece, (N/m²) - q heat flux, (W/m²) - q_f friction heat flux, (W/m²) - heat flux entering towards the roll for any arbitrary element j (W/m²) - R roll radius, (m) - S_o yield stress in plane strain, (N/m²) - T temperature difference (T = T_r – T_o), (°C) - T surrounding temperature, (°C) - y strip thickness, (m) - V_rel relative slipping velocity, (m/sec) - V velocity, (m/sec) - Pe Peclet number - Bi Biot number - _T Total bite angle - mean effective strain - mean true stress, (N/m²) - mean strain rate - friction stress, (N/m²) - coefficient of friction - angle between heating and cooling regions - angle of cooling spray region - r, polar coordinates - x, y Cartesian coordinates - o initial value - f final value - r related to roll - s related to strip - a average value - j elemental region     

On natural convection from a vertical plate with a prescribed surface heat flux in porous media

This paper presents a theoretical and numerical investigation of the natural convection boundary-layer along a vertical surface, which is embedded in a porous medium, when the surface heat flux varies as (1 +x ²)), where is a constant andx is the distance along the surface. It is shown that for > -1/2 the solution develops from a similarity solution which is valid for small values ofx to one which is valid for large values ofx. However, when -1/2 no similarity solutions exist for large values ofx and it is found that there are two cases to consider, namely < -1/2 and = -1/2. The wall temperature and the velocity at large distances along the plate are determined for a range of values of .Notation g Gravitational acceleration - k Thermal conductivity of the saturated porous medium - K Permeability of the porous medium - l Typical streamwise length - q _w Uniform heat flux on the wall - Ra Rayleigh number, =gK(q _w /k)l/(v) - T Temperature - Too Temperature far from the plate - u, v Components of seepage velocity in the x and y directions - x, y Cartesian coordinates - Thermal diffusivity of the fluid saturated porous medium - The coefficient of thermal expansion - An undetermined constant - Porosity of the porous medium - Similarity variable, =y(1+x ) ^/3/x ^1/3 - A preassigned constant - Kinematic viscosity - Nondimensional temperature, =(T – T )Ra^1/3 k/qw - Similarity variable, = =y(log_e x)^1/3/x ^2/3 - Similarity variable, =y/x ^2/3 - Stream function     

变几何域传热的表面热流反演方法

变几何域的表面热流反演是一类特殊的热传导逆问题,在再入飞行器烧蚀型防热材料的表面热流反演中具有工程实用价值.本文首先对变几何域传热的正问题计算方法进行了校核验证,然后建立了求解变几何域表面热流反演问题的顺序函数法和共轭梯度法;给出了这两种反演方法的基本思想和算法推导,并针对典型算例进行了仿真.结果表明:两种反演方法都能计算出较好的反演结果,并且算法受测量噪声的影响较小,具有较好的鲁棒性;反演算法能适应不同的几何域变化函数,但几何域变化量的测量误差在表面热流的反演结果中会有较为直接的反映.     

带人工热通量的移动最小二乘光滑粒子动力学数值方法

叙述了移动最小二乘光滑粒子动力学(MLSPH)的基本原理,讨论了一维MLSPH计算方法,为减小接触间断附近的震荡,在能量方程中,引入了人工热通量,并给出了多介质一维激波管问题的算例,表明了此种方法的有效性。     

Performance enhancement of a DC-operated micropump with electroosmosis in a hybrid nanofluid: fractional Cattaneo heat flux problem

The purpose of this investigation is to theoretically shed some light on the effect of the unsteady electroosmotic flow (EOF) of an incompressible fractional second-grade fluid with low-dense mixtures of two spherical nanoparticles, copper, and titanium. The flow of the hybrid nanofluid takes place through a vertical micro-channel. A fractional Cattaneo model with heat conduction is considered. For the DC-operated micropump, the Lorentz force is responsible for the pressure difference through the microchannel. The Debye-Hükel approximation is utilized to linearize the charge density. The semi-analytical solutions for the velocity and heat equations are obtained with the Laplace and finite Fourier sine transforms and their numerical inverses. In addition to the analytical procedures, a numerical algorithm based on the finite difference method is introduced for the given domain. A comparison between the two solutions is presented. The variations of the velocity heat transfer against the enhancements in the pertinent parameters are thoroughly investigated graphically. It is noticed that the fractional-order parameter provides a crucial memory effect on the fluid and temperature fields. The present work has theoretical implications for biofluid-based microfluidic transport systems.

     

Experimental study on critical heat flux in bilaterally heated narrow annuli

Critical heat flux (CHF) experiments using deionized water as working fluid have been conducted in a range of pressure from 0.6 to 4.2 MPa, mass flow velocity from 60 to 130 kg/m² s and wall heat flux from 10 to 90 kW/m² for vertical narrow annuli with annular gap sizes of 0.95 and 1.5 mm. We found that the CHF, occurring only on the inside tube, or on the outside tube or on both tubes of the annular channel, depends on the heat flux ratio between surfaces of the outside and inside tubes. The CHF, occurring on the surface of the inside tube, reaches the maximum value under the pressure of 2.3 MPa while it occurring on the surface of the outside tube keeps increasing with the increase of the pressure. The CHF, occurring on the inside or outside tubes, increases with the increase of the mass flow velocity and the annular gap size; and decreases with the increase of critical quality and the other tube wall heat flux. Empirical correlations, which agree quite well with the experimental data, have been developed to predict the CHF occurring on surfaces of the inside or outside tubes of the narrow annular channel on the conditions of low pressure and low flow.     

Critical heat flux and turbulent mixing in hexagonal tight rod bundles

Experimental and theoretical investigations have been performed on critical heat flux (CHF) and turbulent mixing in tight, hexagonal, 7-rod bundles. Freon-12 was used as working fluid due to its low latent heat, low critical pressure and well known properties. It has been found that the two-phase mixing coefficient depends mainly on mass flux. It increases with decreasing mass flux and ranges from 0.01 to 0.04 for the test conditions considered. More than 900 CHF data points have been obtained in a large range of parameters: pressure 1.0–3.0 MPa and mass flux 1.0–6.0 Mg/m²s. The effect of different parameters on CHF has been analysed. It has been found that the effect of pressure, mass flux and vapour quality on CHF is similar to that observed in circular tubes. Nevertheless, the CHF in the tight rod bundle is much lower than that in a circular tube of the same equivalent hydraulic diameters. The effect of wire wraps on CHF is mainly dependent on local vapour qualities and subsequently on flow regimes. Based on subchannel flow conditions, the effect of radial power distribution on CHF is small. Comparison of the test results with CHF prediction methods underlines the need for further work.     

Experimental study of critical heat flux enhancement during forced convective flow boiling of nanofluid on a short heated surface

Enhancements of nucleate boiling critical heat flux (CHF) using nanofluids in a pool boiling are well-known. Considering importance of flow boiling heat transfer in various practical applications, an experimental study on CHF enhancements of nanofluids under convective flow conditions was performed. A rectangular flow channel with 10-mm width and 5-mm height was used. A 10 mm-diameter disk-type copper surface, heated by conduction heat transfer, was placed at the bottom surface of the flow channel as a test heater. Aqueous nanofluids with alumina nanoparticles at the concentration of 0.01% by volume were investigated. The experimental results showed that the nanofluid flow boiling CHF was distinctly enhanced under the forced convective flow conditions compared to that in pure water. Subsequent to the boiling experiments, the heater surfaces were examined with scanning electron microscope and by measuring contact angle. The surface characterization results suggested that the flow boiling CHF enhancement in nanofluids is mostly caused by the nanoparticles deposition of the heater surface during vigorous boiling of nanofluids and the subsequent wettability enhancements.     

Characteristics of the critical heat flux for downward flow in a vertical tube at low flow rate and low pressure conditions

The characteristics of the critical heat flux (CHF) for downward flow were studied experimentally with an Inconel 600 circular tube test section in a water test loop at low-flow rate (0 200 kg/m²s) and low-pressure (0.1 0.7 MPa) conditions. The attention was given to the effects of upstream conditions—upper plenum and inlet throttling. Two totally different kinds of CHF behaviors were observed. It seems appropriate to interpret them as flooding-type CHF and dryout in annular flow. The CHF in downward flow may vary from extremely unstable flow CHF as low as near the flooding CHF value to stable flow CHF as high as that of upflow, depending on the upstream conditions of the test section. The CHF correlation by Mishima and that by Weber were proposed for the presentation of the lower and upper limits of the CHF for downward flow in a vertical tube at low-flow rate and low-pressure conditions.     

Natural convection of nanofluid over vertical plate embedded in porous medium: prescribed surface heat flux

The aim of the present paper is to analyze the natural convection heat and mass transfer of nanofluids over a vertical plate embedded in a saturated Darcy porous medium subjected to surface heat and nanoparticle fluxes. To carry out the numerical solution, two steps are performed. The governing partial differential equations are firstly simplified into a set of highly coupled nonlinear ordinary differential equations by appropriate similarity variables, and then numerically solved by the finite difference method. The obtained similarity solution depends on four non-dimensional parameters, i.e., the Brownian motion parameter (N _b), the Buoyancy ratio (N _r), the thermophoresis parameter (N _t), and the Lewis number (Le). The variations of the reduced Nusselt number and the reduced Sherwood number with N _b and N _t for various values of Le and N _r are discussed in detail. Simulation results depict that the increase in N _b, N _t, or N _r decreases the reduced Nusselt number. An increase in the Lewis number increases both of the reduced Nusselt number and the Sherwood number. The results also reveal that the nanoparticle concentration boundary layer thickness is much thinner than those of the thermal and hydrodynamic boundary layers.     

Stagnation point heat flux in hypersonic high enthalpy flow

The paper describes stagnation point heat flux measurements at a range of enthalpies relevant to re-entry speeds of aero-assisted space transfer vehicles (ASTVs) and proposed space planes, using the Australian National University Free Piston Shock Tunnel T3. The unique feature of these experiments is that they were conducted in the straight through (reflectionless) mode which enabled higher enthalpies and densities hitherto unattained.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1992 and the AMS fonts, developed by the American Mathematical Society.