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1.
Electrically heated cylindrical wires are used in research and industry for fluid velocity and turbulence measurements. At very low free-stream velocities (u≤0.1 m/s), hot-wire measurements are significantly influenced by buoyant convection. Below a certain Reynolds number Re* this effect degrades the accuracy of the measurements. To assess the contribution of free-convection heat transfer to the heat balance of hot-wires in cross flow, measurements under normal gravity and microgravity (μg) conditions are compared keeping all other parameters constant. Under gravity conditions, the acceleration of gravity, the hot-wire axis and the direction of the free stream are all perpendicular to each other. The microgravity experiments were carried out in the Drop-Tower Bremen in which the residual acceleration is less than 10−5 g during a period of 4.7 s. The present investigation is concerned with a velocity range of 0≤u≤0.35 m/s corresponding to a Reynolds number range Re<0.1 in standard air. This range includes pure free convection for Re→0 and forced-convection-dominated heat transfer for Re=0.1. At intermediate Reynolds numbers both transport mechanisms must be considered.  相似文献   

2.
H. Ogata  W. Nakayama 《低温学》1977,17(8):461-470
Effect of centrifugal acceleration on heat transfer from the circular flat surface (6.1 mm diameter) of copper to helium was investigated by placing the test piece in a small cell which is revolving about a displaced axis. Free convection data were obtained in the temperature range 4.2–4.5 K, the pressure range 0.1–0.7 MPa (1–7 atm) by imposing centrifugal accelerations of 1-300 ‘g’ towards the surface. Data in the nucleate and film boiling regimes were also obtained at near atmospheric pressures.The new correlation is suggested for free convection heat transfer. However, the existing correlations for conventional fluids in the gravity field seem to be also useful for the design work of superconducting generators, once the gravity term in the Rayleigh number is replaced by a centrifugal acceleration. Nucleate boiling heat transfer is found to be little affected by rotation, although increase in the maximum and minimum heat flux were observed. Also, heat transfer in the film boiling regime was improved.  相似文献   

3.
多壁碳纳米管水基纳米流体的对流换热特性   总被引:1,自引:0,他引:1       下载免费PDF全文
实验研究了纳米粉体浓度、雷诺数Re和热流密度对多壁碳纳米管水基纳米流体(MWNTs/H2O)对流换热性能的影响。纳米粉体浓度分别为0.05 g/L、0.1 g/L、0.2 g/L和0.4 g/L,雷诺数Re为500~900,热流密度为10~20 k W/m2。结果表明:1)纳米流体对流换热系数随着纳米粉体浓度、Re、热流密度的增加而增加。如在Re为631且纳米粉体浓度为0.4 g/L时,纳米流体对流换热系数比基液增大了17.6%;2)纳米流体对流换热系数的提高率明显大于对应的导热系数提高率,当纳米粉体浓度为0.05g/L时,其对流换热系数和导热系数的提高率分别为7.4%和0.15%;3)在Eubank-Proctor方程的基础上,建立了适合于低Re条件下的混和对流换热的实验关联式。  相似文献   

4.
This study deals with heat transfer enhancement surface manufactured by thermal spraying. Two thermal spraying methods using copper as a coating material, wire flame spraying (WFS) and vacuum plasma spraying (VPS), were applied to the outside of copper cylinder with 20 mm OD. The surface structure by WFS was denser than that by VPS. The effect of gravity on boiling heat transfer coeffcient and wall superheat at the onset of boiling were experimentally evaluated under micro- and hyper-gravity condition during a parabolic trajectory flight of an airplane. Pool boiling experiments in saturated liquid of HCFC123 were carried out for heat fluxes between 1.0 and 160 kW/m2 and saturated temperature of 30 °C. As a result, the surface by VPS produced higher heat transfer coefficient and lower superheat at the onset of boiling under microgravity. For the smooth surface, the effect of gravity on boiling heat transfer coefficient was a little. For the coating, a large difference in heat transfer coefficient to gravity was observed in the moderate heat flux range. The heat transfer coefficinet decreased as gravity changed from the normal to hypergravity, and was improved as gravity changed from the hyperto microgravity. The difference in heat transfer coefficient between the normal and microgravity was a little. Heat transfer enhancement factor was kept over the experimental range of heat flux. It can be said that boiling behavior on thermal spray coating might be influenced by flow convection velocity.  相似文献   

5.
A numerical study of conjugated heat transfer in ceiling-slot refrigerated containers is carried out to analyze the temperature distribution effectiveness and to determine the ventilation characteristics. The effect of slot size on thermal characteristics is studied by considering half-span and full-span injection. The container walls are defined as conductive opaque and are interacting with outside environment. The outer surface heat transfer coefficients of conductive walls are computed by studying the flow around the refrigerated truck. The Reynolds number at the slot exit varied between 2 × 104 ≤ Re ≤ 2 × 105. The gravity effect is taken into account, and the coupled mass, momentum, and energy equations are discretized in finite volumes. The heat transfer coefficients of inner flow are presented as plots of the mean Nusselt number versus the modified Reynolds number. The maximum dispersion in the numerical data being at 14.54-percent, the mean Nusselt number, the modified Reynolds number, and the aspect ratio of the container are correlated.  相似文献   

6.
The flow boiling heat transfer characteristics of subcooled air-dissolved FC-72 on a smooth surface (chip S) were studied in microgravity by utilizing the drop tower facility in Beijing. The heater, with dimensions of 40 × 10 × 0.5 mm3 (length × width × thickness), was combined with two silicon chips with the dimensions of 20 × 10 × 0.5 mm3. High-speed visualization was used to supplement observation in the heat transfer and vapor-liquid two-phase flow characteristics. In the low and moderate heat fluxes region, the flow boiling of chip S at inlet velocity V =?0.5 m/s shows almost the same regulations as that in pool boiling. All the wall temperatures at different positions along the heater in microgravity are slightly lower than that in normal gravity, which indicates slight heat transfer enhancement. However, in the high heat flux region, the pool boiling of chip S shows much evident deterioration of heat transfer compared with that of flow boiling in microgravity. Moreover, the bubbles of flow boiling in microgravity become larger than that in normal gravity due to the lack of buoyancy Although the difference of the void fraction in x-y plain becomes larger with increasing heat flux under different gravity levels, it shows nearly no effect on heat transfer performance except for critical heat flux (CHF). Once the void fraction in y-z plain at the end of the heater equals 1, the vapor blanket will be formed quickly and transmit from downstream to upstream along the heater, and CHF occurs. Thus, the height of channel is an important parameter to determine CHF in microgravity at a fixed velocity. The flow boiling of chip S at inlet velocity V =?0.5 m/s shows higher CHF than that of pool boiling because of the inertia force, and the CHF under microgravity is about 78–92% of that in normal gravity.  相似文献   

7.
The combustion of a flat plate in a boundary layer under microgravity conditions, which was first described by Emmons, is studied using a gas burner. Magnitude of injection and blowing velocities are chosen to be characteristic of pyrolyzing velocity of solid fuels, and of ventilation systems in space stations. These velocities are about 0.1 m/s for oxidiser flow and 0.004m/s for fuel flow. In this configuration, flame layout results from a coupled interaction between oxidiser flow, fuel flow and thermal expansion. Influences of these parameters are studied experimentally by means of flame length and standoff distance measurements using CH* chemiluminescence’s and visible emission of the flame. Flow was also studied with Particle Image Velocimetry (PIV). Inert flows, with and without injection, and reacting flow in a microgravity environment were considered to distinguish aerodynamic from thermal effect. Thermal expansion effects have been shown by means of the acceleration of oxidiser flow. Three-dimensional effects, which are strongly marked for high injection velocities were studied. Three-dimensional tools adaptability to parabolic flights particular conditions were of concern. Flame sensitivity to g-jitters was investigated according to g-jitters frequency and range involved by parabolic flights. It appears that flame location (standoff distance), flame characteristics (length, thickness, brightness) and the aerodynamic field of the low velocity reacting flow are very much affected by the fluctuation of the gravity level or g-jitter. The lower the g-jitter frequency is, the higher the perturbation. Consequently it is difficult to perform relevant experiments for a main flow velocity lower than 0.05m/s. DNS calculations confirm the present observations, but most of the results are presented elsewhere.  相似文献   

8.
Bubble Dynamics in Nucleate Pool Boiling on Thin Wires in Microgravity   总被引:2,自引:0,他引:2  
A temperature-controlled pool boiling (TCPB) device has been developed to study the bubble behavior and heat transfer in pool boiling phenomenon both in normal gravity and in microgravity. A thin platinum wire of 60 μm in diameter and 30 mm in length is simultaneously used as heater and thermometer. The fluid is R113 at 0.1 MPa and subcooled by 26°C nominally for all cases. Three modes of heat transfer, namely single-phase natural convection, nucleate boiling, and two-mode transition boiling, are observed in the experiment both in microgravity aboard the 22nd Chinese recoverable satellite and in normal gravity on the ground before and after the space flight. Dynamic behaviors of vapor bubbles observed in these experiments are reported and analyzed in the present paper. In the regime of fully developed nucleate boiling, the interface oscillation due to coalescence of adjacent tiny bubbles is the primary reason of the departure of bubbles in microgravity. On the contrary, in the discrete bubble regime, it’s observed that there exist three critical bubble diameters in microgravity, dividing the whole range of the observed bubbles into four regimes. Firstly, tiny bubbles are continually forming and growing on the heating surface before departing slowly from the wire when their sizes exceed some value of the order of 10−1 mm. The bigger bubbles with about several millimeters in diameter stay on the wire, oscillate along the wire, and coalesce with adjacent bubbles. The biggest bubble with diameter of the order of 10 mm, which was formed immediately after the onset of boiling, stays continuously on the wire and swallows continually up adjacent small bubbles until its size exceeds another critical value. The same behavior of tiny bubbles can also be observed in normal gravity, while the others are observed only in microgravity. Considering the Marangoni effect, a mechanistic model about bubble departure is presented to reveal the mechanism underlying this phenomenon. The predictions are qualitatively consistent with the experimental observations.  相似文献   

9.
Marangoni thermocapillary convection and its contribution to heat transfer during boiling has been the subject of some debate in the open literature. Despite extensive research efforts there still remains insufficient quantitative information regarding the impact of thermocapillary flow on the heat transfer. As a result, this paper aims to present a numerical investigation of the heat transfer enhancement due to Marangoni thermocapillary convection under both earth gravity (1-g) and zero gravity (0-g) conditions. A hemispherical bubble of fixed shape is considered atop a heated top wall of a domain with variable height. The heat transfer enhancement is quantified for Marangoni numbers in the range of 100 ≤ Ma ≤ 3,000 for channel heights of 1.5 ≤ H/Rb?≤ 7.5 which, for the 1-g cases, correspond with a Raleigh number range of 51 ≤ RaH?≤ 6.5 × 104. For the most confined cases the flow and heat transfer were found to be very similar for the 0-g and 1-g cases. Also, the 0-g test cases were found to be very sensitive to increasing domain height whereas the 1-g simulations were far less sensitive.  相似文献   

10.
Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm3) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.  相似文献   

11.
An analysis is presented to study the effects of transpiration on free convection in an annulus between concentric porous spheres. A regular perturbation technique is applied to solve the steady-state Navier-Stokes equations of motion and the energy equation, whereby the stream-function and temperature are expanded in the form of power series in terms of the Rayleigh number Ra, and injection/suction Reynolds number Re. The analytical solution is valid for small values of Ra and Re, and all values of the Prandtl number Pr. A finite-difference solution of the governing steady-state equations of motion and energy is also provided. The range of validity of the analytical solution is determined by comparison with the numerical solution. Plots of the flow patterns, velocity distributions, temperature profiles, local and overall heat transfer rates are presented for Pr = 0.7 and different values of Re, Ra, and the radius ratio . Results are shown over a range of values of Ra and Re such that the effects of mixed convection in the annulus can be clearly observed.  相似文献   

12.
The paper considers the conditions that are necessary to secure accurate measurement of the apparent thermal conductivity of two-phase systems comprising nanoscale particles of one material suspended in a fluid phase of a different material. It is shown that instruments operating according to the transient hot-wire technique can, indeed, produce excellent measurements when a finite element method (FEM) is employed to describe the instrument for the exact geometry of the hot wire. Furthermore, it is shown that an approximate analytic solution can be employed with equal success, over the time range of 0.1 s to 1 s, provided that (a) two wires are employed, so that end effects are canceled, (b) each wire is very thin, less than 30 \(\upmu \)m diameter, so that the line source model and the corresponding corrections are valid, (c) low values of the temperature rise, less than 4 K, are employed in order to minimize the effect of convection on the heat transfer in the time of measurement of 1 s, and (d) insulated wires are employed for measurements in electrically conducting or polar liquids to avoid current leakage or other electrical distortions. According to these criteria, a transient hot-wire instrument has been designed, constructed, and employed for the measurement of the enhancement of the thermal conductivity of water when TiO\(_{2}\) or multi-wall carbon nanotubes (MWCNT) are added. These new results, together with a critical evaluation of other measurements, demonstrate the importance of proper implementation of the technique.  相似文献   

13.
This paper presents a direct numerical simulation of the vortical structure along the bottom wall and its effect on heat transfer enhancement in the transitional flow over a backward-facing step. The governing equations are solved with a computer program using FORTRAN code according to finite volume method. The heat transfer of the bottom wall is enhanced greatly for the transitional Reynolds numbers, especially in the range 6 ≤ x/H ≤ 14. In this range, the local Nusselt number distributions for various time instants of the periodic cycle exist three peak values at Re = 1200. The corresponding locations of Nu peak values in the vorticity contours are all between the negative vortex and the adjacent downstream positive vortex. This vortical structure appears due to the flow instability which causes the heat transfer enhancement of the bottom wall in the transitional flow.  相似文献   

14.
An investigation has been made of special features of heat transfer from horizontal wires under combined forced and free convection. It is shown that in counterflow, in contrast with auxiliary flow, the dependence of Nu(Re)Ra=const bas a minimum. Near the minimum in the range 0.15 < Re2/Ra√Re < 1.5 there is a region where the heat transfer conditions are oscillatory.  相似文献   

15.
Experiments on heating titanium wire specimens 0.5 mm in diameter by electric current in 0.5 absolute atmospheric pressure (0.5 kg/cm2) of a pure nitrogen gas environment under normal gravity (g = 9.8 m/s2 = 1 G) and microgravity (g = G) using a drop shaft that enables 10 s of a microgravitational condition were carried out to investigate fundamentally the influence of microgravity on the reaction between Ti and nitrogen gas. The influence of gravitational acceleration on the morphological changes in optical microscopic microstructures of the specimens was studied. Under normal gravity, the thickness of the TiN layer resulting from the reaction of Ti and nitrogen gas at the outermost surface of the specimen increased with the heating time, but the tendency for the increase was reduced. A Widmanstätten structure, which is made by rapid heating and cooling of the specimen, was observed. However, the typical characteristics of the Widmanstätten structure vanished as the heating time proceeded. The progress of the nitriding reaction of the specimen was reduced, because heating by the electrical current and cooling by the convection of nitrogen gas balance each other out, and the temperature of the specimen became constant. Under microgravity, on the other hand, the thickness of the TiN layer of the outermost surface of the specimen increased with the heating time and was always thicker than that under normal gravity. The influence of microgravity, which suppresses the cooling effect of the thermal convection of nitrogen gas, on the nitriding reaction between titanium and nitrogen gas was significant. A dendritic structure caused by electrical heating was observed. Since the combustion synthesis of TiN was initiated at an elevated temperature after a certain heating time, huge dendritic structures were observed. The TiN tube was made on a final stage by leaching the titanium from the specimen. The influence of microgravity on the microstructure of the specimens that underwent nitriding was notable. Various results of the present study are explained by gravitational acceleration effects. The nitriding reaction became active by a temperature increase in the specimens with heating time due to the suppression of thermal convection and the cooling effect of the nitrogen gas on the specimens was reduced.  相似文献   

16.
This paper describes the results of a sounding rocket experiment which was partly dedicated to study the heat transfer from a hot wall to a cold liquid with a free surface. Natural or buoyancy-driven convection does not occur in the compensated gravity environment of a ballistic phase. Thermo-capillary convection driven by a temperature gradient along the free surface always occurs if a non-condensable gas is present. This convection increases the heat transfer compared to a pure conductive case. Heat transfer correlations are needed to predict temperature distributions in the tanks of cryogenic upper stages. Future upper stages of the European Ariane V rocket have mission scenarios with multiple ballistic phases. The aims of this paper and of the COMPERE group (French–German research group on propellant behavior in rocket tanks) in general are to provide basic knowledge, correlations and computer models to predict the thermo-fluid behavior of cryogenic propellants for future mission scenarios. Temperature and surface location data from the flight have been compared with numerical calculations to get the heat flux from the wall to the liquid. Since the heat flux measurements along the walls of the transparent test cell were not possible, the analysis of the heat transfer coefficient relies therefore on the numerical modeling which was validated with the flight data. The coincidence between experiment and simulation is fairly good and allows presenting the data in form of a Nusselt number which depends on a characteristic Reynolds number and the Prandtl number. The results are useful for further benchmarking of Computational Fluid Dynamics (CFD) codes such as FLOW-3D and FLUENT, and for the design of future upper stage propellant tanks.  相似文献   

17.
This paper deals with a similarity analysis of axisymmetric mixed convection between two horizontal infinite rotating coaxial disks in the presence of a magnetic field. The governing Navier-Stokes equations and energy equation reduce to a set of nonlinear ordinary differential equations using similarity. The resulting set of ordinary differential equations has been solved numerically using a shooting method and is represented graphically. For Reynolds number, Re, up to 500 and buoyancy parameter, B=βΔT, of the range |B| ≤0.05, the flow and heat transfer characteristics with Prandtl numbers 7, 0.7, 0.1 and 0.01 and magnetic field parameters of m=0.5, 1, and 2 are examined. The heat transfer increases with the Prandtl number but decreases with the magnetic parameter. It is established that the rotation of the two disks has a very small effect on the temperature of the fluid and the heat-transfer process. Published in Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 5, pp. 1109–1117, September–October, 2000.  相似文献   

18.
以SK型翅片管式换热器为研究对象,在循环式风洞中对其结霜工况下的性能进行试验研究,研究了入口空气流速和相对湿度等环境参数对SK型翅片管式换热器性能的影响。研究结果表明:结霜工况下,翅片表面未覆盖满霜层时,在雷诺数Re=3602~5509,进口相对湿度?=60%~80%范围内,空气侧对流换热系数随迎面风速的增大而增大,随相对湿度的增加而增加;换热器表面阻力降随着流速的增加而增大,随着相对湿度的增大而增大。实验结果表明,空气相对湿度对SK型翅片管式换热器性能的影响远大于空气流速的影响。  相似文献   

19.
Inflow boiling, gravity effects on the distribution of both phases are observed in a heated tube and heat transfer coefficients due to two-phase forced convection is deteriorated in microgravity. In narrow channels between heated and unheated plates, the increase in subcooling enlarges a size of flattened bubble and reduces the frequency of detachment under microgravity conditions resulting the emphasis of heat transfer deterioration. To clarify reasons for the unknown behaviors of interfacial distribution and corresponding characteristics in heat transfer not easily be clarified through the experiments on ground, the opportunity on the experiments utilizing long-term microgravity duration realized in ISS is required. The experiments on microgravity boiling and two-phase flow are proposed by the collaboration of researchers in five countries. A common test loop is designed to conduct multiple experiments by the interchangeable structures of test sections; a transparent heated tube for the visualized flow boiling, a stainless tube for the measurement of CHF data, a copper surface for the heat transfer data of nucleate boiling with superimposed liquid flows in a duct, a glass heated plate with multiple array of small temperature sensors and transparent heaters for the clarification of mechanisms in nucleate boiling heat transfer, and one or two models of cold plates for practical applications. A direction of researches in the present discipline is proposed based on the existing experimental results and on the idea developed by the present authors.  相似文献   

20.
Convective heat transfer is governed by a number of factors including various fluid properties, the presence of a thermal gradient, geometric configuration, flow condition, and gravity. Empirically-derived analytical relationships can be used to estimate convection as a function of these governing parameters. Although it is relatively straightforward to experimentally quantify the contributions of the majority of these variables, it is logistically difficult to assess the influence of reduced-gravity due to practical limitations of establishing this environment. Therefore, in order to explore this regime, a series of tests was conducted to evaluate convection under reduced-gravity conditions averaging 0.45 m/sec2 (0.05 g) achieved aboard a parabolic aircraft. The results showed a reduction in net heat transfer of approximately 61% in flight relative to a 1g terrestrial baseline using the same setup. The average experimental Nusselt Number of 19.05 ± 1.41 statistically correlated with the predicted value of 18.90 ± 0.63 (N = 13), estimated using the Churchill-Chu correlation for free convective heat transfer from a finite, flat, vertical plate. Extrapolating this to similar performance in true microgravity (10?6 g) indicates that these conditions should yield a Nusselt Number of 1.27, which is 2.6% the magnitude of free convection at 1g, or a reduction of 97.4%. With advection essentially eliminated, heat transfer becomes limited to diffusion and radiation, which are gravity-independent and nearly equivalent in magnitude in this case. These results offer a general guideline for integrating components that utilize natural (free) convective gas cooling in a spacecraft habitat and properly sizing the thermal control system.  相似文献   

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