Flow and heat transfer behaviors of phase change material slurries in a horizontal circular tube

The flow and convective heat transfer behaviors of microencapsulated phase change material (MPCM) slurries in a horizontal circular tube have been experimentally investigated. The slurry consisted of microencapsulated 1-bromohexadecane (C₁₆H₃₃Br) and water, with the mass fractions of MPCM varying from 5% to 27.6%. The pressure drop and local heat transfer coefficients were measured, and the influences of capsule fractions, heating rates, and flow structures on heat transfer performance were also studied. Heat transfer coefficients measured for MPCM slurry are significantly higher than for those for single-phase fluid flow in laminar flow conditions, but exhibit more complicated phenomena at low turbulent conditions. Moreover, a new simple heat transfer correlation equation was proposed that accurately predicts the local heat transfer coefficients of laminar MPCM slurry flow in a horizontal circular tube.     

Influence of a pulsation on heat transfer and flow structure in submerged impinging jets

An experimental investigation on pulsating impinging jets has been performed. The effect of the pulsation on the flow structure and heat transfer have been investigated. Frequency and amplitude were varied separately and the effect of each parameter was examined for different Reynolds numbers and nozzle-to-plate distances.The jet was found to become broader and the core jet length smaller with the pulsation. The reason for this behavior is that pulsation enhanced entrainment of air into the jet, which results in a change of mean velocity of the jet. Nevertheless, the behavior at lower frequencies (up to 140 Hz) is still quasisteady. This means that the amplitude of the pulsation behaves similar to the mean velocity of the jet, that the shapes of the velocity profiles are comparable to steady jets and that the jet behavior is independent of frequency.At moderate frequencies heat transfer is only affected by the pulsation when nozzle-to-plate distance and amplitude are large enough. At small nozzle-to-plate distances enhanced entrainment has no influence and no difference between steady and pulsating jets can be recognized. At large nozzle-to-plate distances entrainment increases and jet velocity reduces. This yields a reduction of heat transfer in the stagnation point of up to 50%.But besides of this effect of enhanced entrainment a theoretical limit could be determined, above which the jet is not anymore quasisteady. Above Sr = 0.2 heat transfer is affected by the pulsation also at small nozzle-to-plate distances. At this frequency boundary layer is also affected by the pulsation. This yields increased heat transfer coefficients at the stagnation point. For larger nozzle-to-plate spacings this effect is superposed by the reduction of heat transfer due to increased entrainment, resulting in a strong decrease of heat transfer coefficient.     

Laminar heat transfer and friction characteristics of microencapsulated phase change material slurry in a circular tube with twisted tape inserts

It is novel and better method that microencapsulated phase change material (MPCM) slurry and the tube with twisted tape inserts are adopted together to enhance convective heat transfer. In this paper, numerical analyses were carried out to study laminar heat transfer and friction characteristics of MPCM slurry in a circular tube with twisted tape inserts. It is found that the MPCM slurry in the tube with twisted tape insert leads to the best performance of convective heat transfer for the bigger apparent specific heat and the intensive swirl flow. Furthermore, the modified average Nusselt number increases with decreasing bulk Stefan number, twisted ratio and increasing Re, while the friction factor increases with increasing Re and decreasing twisted ratio. It is also found that the heat transfer enhanced efficiency increases with increasing Re and decreasing Ste_b, and the heat transfer enhancement effects of twisted tape for low Ste_b slurry are better than that for high Ste_b slurry. Moreover, the thermal-hydraulic performance ratios increase to a peak, then decrease gradually with increasing Re for different twist ratio. The performance ratio increases with decreasing the twist ratio only in a definite Re range, and the Re range decreases with decreasing twist ratio.     

Experimental investigation on frequency pulsation effects on a single pass plate heat exchanger performance

This paper investigates combined heat transfer improvement methods. These methods include introducing pulsating flow, adding nanofluids, and manipulating the flow's characteristics in a corrugated plate heat exchanger. Tests are carried out with multi-walled carbon nanotube (MWCNT), graphene nanoplate (GNP), and a mixture of GNP and MWCNT meeting the requirement of 0.01% nanofluids volume fraction and exposed to pulsation. Results demonstrated that the use of pulsating frequencies from 0 to 30 Hz of GNP-water, MIX nanofluids–water, and MWCNT–water nanofluids with a constant concentration of 0.01 wt% leads to a significant improvement in heat transfer. Using pure water at frequency f = 0 Hz as a benchmark, the Nusselt number of the mixture nanofluid increases by 15.2%, 27.5%, 40.4%, and 52.8% with the increase of frequency pulsation from 0 to 30 Hz with a slight effect on the pressure-drop at this low used constant nanofluid concentration = 0.01%. The highest Nusselt number value for GNP-water nanofluid improved by an amount of 58.3% at the highest frequency compared with pure water at f = 0 Hz.     

Thermal performance of microencapsulated phase change material slurry in turbulent flow under constant heat flux

Current cooling and heating distribution systems that use water as secondary fluid exhibit limited thermal capacity which can only be overcome by high flow rates and large (volume) capacity. A successful way to enhance the thermal capacity of secondary fluid systems is by incorporating microencapsulated phase change material (MPCM) slurry. However, a full understanding of the physical properties and heat transfer characteristics of MPCM slurry in the 2–8 °C range (35.5–46.5 °F) still is lacking. In the paper, latent heat of fusion, melting and freezing points, and temperature- and concentration-dependent viscosity data, are presented. Results indicate that selection of nucleating agent type and concentration is required to prevent the supercooling phenomenon. Pressure drop and convective heat transfer data were measured using a heat transfer loop operated at different flow rates and heat flux values. Results indicate that the phase change process and slurry mass fraction affect the heat transfer process. The paper also examines the impact of using enhanced surface tubing in combination with MPCM slurry under constant heat flux and turbulent conditions.     

恒壁温下管内流体脉动流动对流换热的数值模拟

利用FLUENT软件,在流体进口速度发生周期性变化的情况下,对圆管内层流的流体流动与换热情况进行了数值模拟,分析了无因次脉动幅值、频率对对流换热系数、摩擦系数以及流场的影响,结果表明脉动流体会强化或弱化换热效果,阻力比无脉动时大,并且在流场中有与主流区流动方向相反的流动现象。     

Heat transfer characteristics in a small-scale fluidized bed boiler

Heat transfer characteristics in a small-scale fluidized bed boiler (2MW_th) were studied using lignite and corn cob as fuels. Depending on air velocity, the heat transfer rates from bed to water membrane wall and from hot flue gas to convective tube bank were in the ranges 75–55% and 25–45% of the total heat absorbed by the boiler, respectively. At designed capacity, the heat transfer flux based on bed cross sectional area and on water membrane wall area were about 0·45 and 0·15 MWm⁻², respectively. Under the conditions studied, it was found that the overall heat transfer coefficient between bed and water membrane wall was 100–300 W m⁻² K⁻¹, whereas that between flue gas and convective tube bank was 10–30 Wm⁻² K⁻¹. The study of heat transfer to a horizontal tube immersed in the bed as well as placed in the freeboard region were also studied. The effective heat transfer coefficients were found to be 300–800 W m⁻² K⁻¹ for in-bed tube and 30–150 W m⁻² K⁻¹ for the freeboard region, depending on air velocity. Comparison of these data with those predicted by both modelling and correlation reported in the literature was also made. For the immersed tube, good agreement was observed for low air velocity, while at high air velocity the experiment produced results twice those estimated from modelling and correlation. For the freeboard region, the model gave a fair prediction.     

Heat transfer downstream of a 3D confined square cylinder under flow pulsation

Abstract

This 3D study deals with the effect that a low Reynolds number (Re) pulsating water flow has on both the heat transfer to the walls of a square section channel in which a square section prism was located, and the associated pumping power. The prism blockage ratio was 2.5/1. Re, based on the prism cross-section height, varied from 100 up to 200. The inflow velocity profile was sinusoidal. Taking St₀ as the Strouhal number of the vortex shedding downstream of the prism at Re 100, the inflow Strouhal of the pulsating cases varied between 0.125 St₀ and 4 St₀. The results show that, for this particular geometry and parametric space, flow pulsation generally enhances heat transfer although its extent depends on Re and pulsation frequency. In the most favorable case, heat transfer doubles as compared to the clean channel case at the expense of tripling the pumping power.     

填充床内对流换热的数值研究

为研究空气流入高温填充床时小球直径和空气流速变化对填充床内对流换热和压力损失等的影响,利用孔隙尺度介观方法对顺序排列多孔介质小球的三维填充床进行数值计算,数值计算与实验结果吻合较好。结果表明:填充床内固相和气相间存在热的非平衡性;当小球直径从2.8增大到5.6 mm时,在最高温度上游对流换热强度减小,在最高温度下游对流换热强度增大,同时,压力损失和最大无量纲速度减小;气体流速增大时,填充床内产生湍流运动。     

An Experimental Investigation of Heat Transfer Enhancement Mechanisms in Microencapsulated Phase-Change Material Slurry Flows

This article investigates laminar heat transfer characteristic of two-phase microencapsulated phase-change material (MPCM) suspension flows within minichannels under a constant wall heat flux boundary. Capsules containing paraffin wax with phase-change temperature between 36.1°C and 38.1°C are examined and found to be well suited for electronics cooling applications using liquid cold plate technologies. In particular, it is shown that the large thermal capacity of MPCM slurries around the phase-change temperature can lead toward greater isothermality of isoflux systems, a characteristic of significant interest to telecommunication, laser and biomedical applications. The principal focus of the study is to examine heat transfer characteristics within standard tube flow geometries, quantify the heat transfer augmentation/degradation observed, and finally, elucidate the mechanisms from which these result. Through the study volume concentrations of the MPCM slurry were varied between 0% and 30.2%. High-resolution local heat transfer measurements were obtained using infrared thermography and results presented in terms of local Nusselt number versus inverse Graetz parameter. These spanned both the thermal entrance and the fully developed flow regions with inverse Graetz number ranging from 10^?3 to 10⁰. Results show that significant heat transfer enhancements are attainable via the use of MPCM slurries over conventional single-phase coolants. Overall, the study highlights mechanisms that lead to significant heat transfer enhancements in heat exchange devices employing microencapsulated phase-change material slurries.     

Flow of Microencapsulated Phase Change Material Slurry through Planar Spiral Coil

Forced convection cooling is an effective method in thermal management that relies mainly on dissipating heat by pumping heat transfer fluid (HTF) through the heat source. In this paper, we investigate the thermal properties enhancement of dielectric water as the HTF. To enhance the properties of the HTF, microencapsulated phase change materials (MPCM) will be added to the base fluid. The MPCMs are composed of phase change material (PCM) encapsulated with shell materials. The PCM inside the capsules may undergo a phase change. This leads to a significant heat gain and release. The numerical model is developed to solve for continuity, momentum, and heat transfer equations using the finite volume method. The behavior of the MPCM slurry in curved channels, generates unique patterns due to different viscosity values and the centrifugal forces. Our preliminary numerical data on MPCM slurry through planar spiral coil heat exchangers show the new patterns of velocity and heat transfer curves. The current paper studies the steady condition of laminar flow at different boundary conditions. The velocity and temperature profiles, heat transfer data with different mass fractions of MPCM additives to the base fluid, and their heat removal capabilities are quantified and discussed in detail.     

Turbulent impinging jet heat transfer enhancement due to intermittent pulsation

A numerical study was carried out of heat transfer under a pulsating turbulent slot impinging jet. The jet velocity was varied in an intermittent (on–off) fashion. The effects of the time-mean jet Reynolds number, temperature difference between the jet flow and the impinging surface, nozzle-to-target distance as well as the frequency on heat and mass transfer were examined. The numerical results indicate significant heat transfer enhancement due to intermittent pulsation of the jet flow over a wide range of conditions for both cooling and heating cases. Simulations of the flow and temperature fields show that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.     

Numerical analysis on the heat and mass transfer MHD flow characteristics of nanofluid on an inclined spinning disk with heat absorption and chemical reaction

This work examines the heat transfer properties of magnetohydrodynamic nanofluid flow. Through a similarity conversion, the leading structure of partial differential equations is changed to that of ordinary differential equations. A rigorous mathematical bvp4c methodology is used to generate numerical results. The purpose of this study is to characterize the different temperature, concentration, and velocity limitations on a nanofluid with a magnetic effect that is spinning. The findings for rotating nanofluid flow and heat transfer characteristics of nanoparticles are shown using graphs and tables. The influence of physical factors such as heat transfer rates and skin friction coefficients is studied. When the magnetic parameter M is raised, the velocity of the nanoliquid decreases. A rise in thermal radiation (Rd) causes the temperature graphs to grow substantially, although the concentration profiles exhibit the opposite tendency. The effect of the convective heat transfer factor Bi on temperature is shown to increase as Bi increases, but the concentration distribution decreases as Biot increases.     

Heat transfer in a crater bed

The aim of this work is to study heat transfer in a laboratory scale crater bed, which was set up from a cylindrical acrylic/quartz tube, using sand as the bed particle. The bed employs a downward gas jet from a nozzle which causes the particles to ascend fountain-like into the freebroad, leaving a crater on the bed surface. After reaching a certain height, these particles will descend again to the bed surface and move into the crater, where the cycle or circulation pattern starts again. The study had been separated into three parts. Firstly, the void fraction of the bed fountain zone was studied by direct measurement of the ascending sand weight within the specific volume. Secondly, the convection heat transfer coefficients between the fountain zone and the external surface of the gas inlet tube were determined by measuring the quantity of heat loss from an electrical heater that was wrapped on the outside surface at desired positions of the gas inlet tube. Thirdly, the radiation heat transfer coefficients were evaluated by heat balance of LPG combustion in the crater bed. From experimental results, the void fraction of the fountain zone could be approximated as a dilute bed (>0.98). For convective heat transfer coefficients, the value found experimentally varied from 80–260 W/m² K depending on the experimental conditions, showing an increase when the gas velocity increases, and a decrease along the height of the gas inlet tube. Radiation heat transfer coefficients, the values of which are (within the experimental temperature range), the same order as the convective mode, increase when the bed temperature is increased and when the bed particle diameter is decreased. Empirical correlations for both bed voidage and heat transfer coefficients are proposed. The combined model, gas and particle convection and the published data on radiation heat transfer, showed good prediction when compared with experimental data.     

Flow and heat transfer characteristics in pulsating pipe flows (effects of pulsation on internal heat transfer in a circular pipe flow)

Effects of pulsation on flow and heat transfer characteristics are experimentally examined in the pulsating pipe flows having sinusoidal velocity fluctuations around a nonzero mean. By systematically varying three pulsation parameters (the amplitude, frequency, and mean velocity), time-averaged and fluctuating temperature profiles are measured under the heating condition of constant wall temperature using saturated vapor. The mean Nusselt number, Nu_p, is calculated, and compared with that in ordinary turbulent pipe flows without pulsation. The results show that Nu_p, decreases initially as the pulsation amplitude increases, then recovers gradually, and finally becomes much greater than the original value. In pulsating pipe flows with a nonzero mean velocity, therefore, pulsation cannot always promote heat transfer, but sometimes suppresses it, depending mainly on the pulsation amplitude and mean velocity. It is also found that these heat transfer characteristics of a pulsating pipe flow are controlled by the transition of flow patterns with pulsation amplitude from a fully turbulent flow to a conditionally turbulent flow via a transitional flow. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(5): 323–341, 1996     

Heat transfer characteristics in a pulsating fluidized bed in relation to bubble characteristics

A pulsating fluidized bed is operated with two sequential durations designated as an on‐period with injecting fluidization gas and an off‐period without it. The heat transfer coefficient between a vertically immersed heater and bed in a pulsating fluidized bed is measured under various pulse cycles and fluidized particles. The obtained results are compared with those in a normal fluidized bed with continuous fluidization air injection. The relationship between heat transfer coefficients and bubble characteristics, evaluated using a digital video camera, has also been investigated. For certain fluidized particles and operating pulse cycles, the fluidization of particles and the increment of heat transfer coefficients can be obtained under a mean air velocity based on a pulse cycle duration smaller than the minimum fluidization air velocity in a normal fluidized bed. Under the pulse cycles where a static bed through the whole bed is formed in the off‐period duration, the improved heat transfer rate over that in a normal fluidized bed can be measured. This may be attributed to large bubble formation. As heat transfer in the pulsating fluidized bed is obstructed with increasing time to keep a static bed due to the excessive off‐period duration, it is indicated that there is an optimum off‐period duration based on the heat transfer rate. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 307–319, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10038     

An analytical study of pulsating laminar heat convection in a circular tube with constant heat flux

Pulsating laminar convection heat transfer in a circular tube with constant wall heat flux is investigated analytically. The results show that both the temperature profile and the Nusselt number fluctuate periodically about the solution for steady laminar convection, with the fluctuation amplitude depending on the dimensionless pulsation frequency, ω*, the amplitude, γ, and the Prandtl number, Pr. It is also shown that pulsation has no effect on the time-average Nusselt numbers for pulsating convection heat transfer in a circular tube with constant wall heat flux.     

Mixing characteristics of pulsed air-assist liquid jet into an internal subsonic cross-flow

Penetration depth,spray dispersion angle,droplet sizes in breakup processes and atomization processes are very important parameters in combustor of air-breathing engine.These processes will enhance air/fuel mixing inside the combustor.Experimental results from the pulsed air-assist liquid jet injected into a cross-flow are investigated.And experiments were conducted to a range of cross-flow velocities from 42～136 m/s.Air is injected with 0～300kPa,with air-assist pulsation frequency of 0～20Hz.Pulsation frequency was modulated by solenoid valve.Phase Doppler Particle Analyzer(PDPA) was utilized to quantitatively measuring droplet characteristics.High-speed CCD camera was used to obtain injected spray structure.Pulsed air-assist liquid jet will offer rapid mixing and good liquid jet penetration.Air-assist makes a very fine droplet which generated mist-like spray.Pulsed air-assist liquid jet will introduce additional supplementary turbulent mixing and control of penetration depth into a cross-flow field.The results show that pulsation frequency has an effect on penetration,transverse velocities and droplet sizes.The experimental data generated in these studies are used for a development of active control strategies to optimize the liquid jet penetration in subsonic cross-flow conditions and predict combustion low frequency instability.     

船用低速柴油机燃油喷射控制阀的性能仿真与结构优化

以自主研制的电控液力增压式喷油器中的燃油喷射控制电磁阀为研究对象,在LMS-AMESim中搭建了电磁阀液力仿真模型,在ANSYS-Maxwell中搭建了电磁铁仿真模型,并通过AMESim中的电磁铁查询模型对Maxwell中的电磁铁数据进行耦合.经电磁阀综合性能试验平台测试,验证了所建模型的精度满足要求.利用该模型仿真分...     

Effects of hall current and wall temperature oscillation on free convective and mass transfer flow in a rotating porous medium with constant heat source

The combined effects of Hall current and a constant heat source on the hydromagnetic free convective and mass transfer flow past an infinite vertical porous plate in a rotating porous medium are considered, when the temperature of the plate varies with time about a nonzero constant mean and the temperature of the free stream is constant. The problem is solved analytically and the velocity profiles are shown on graphs. Effects of m (Hall parameter) and α (heat source parameter) on velocity are discussed extensively.