Laminar flow and heat transfer of non-newtonian falling liquid film on a horizontal tube with variable surface heat flux

An analysis is performed to study the laminar flow and heat transfer of non-Newtonian falling liquid film on a horizontal tube for the case of variable surface heat flux. The inertia and convection terms are taken into account. The governing boundary layer equations are solved numerically using an implicit finite difference method. Of particular interest are the effects of the mass flow rate Γ, the concentration C of carboxymethylcellulose (CMC) solutions, the exponent m for the power-law surface heat flux, and the tube diameter D on the film thickness profiles, as well as on the local and average Nusselt numbers. It was found that an increase in the mass flow rate Γ and exponent value m increases the local and average heat transfer rates. Finally, the present simulation is found to be in good agreement with previous experimental and numerical results for Newtonian films.     

An experimental investigation into a solar assisted desiccant-evaporative air-conditioning system

In this experimental investigation a solar assisted open adsorption cooling system has been designed and tested under the local weather conditions of Basrah, Iraq. Data were obtained from June to September, inclusive, 1984. Tests were carried out hourly with a directly supply of hot air from a corrugated absorber solar air heater for regeneration. Also, tests were conducted at a constant regeneration temperature of 70°C using auxiliary heat. Adsorption was carried out by a rotary disk of silica gel. Three mass flow rates of process air were employed without recirculation. The performance of the solar air heater was obtained for both seasons, and the instantaneous efficiency was evaluated experimentally and analytically with results compared. Daily and seasonal coefficients of performance were obtained for the cooling system for the mass flow rates employed. A maximum seasonal average value of 2.8 was obtained for a mass flow rate of 0.075 kg/s. The system performance improved with higher regeneration temperature, higher process air mass flow rate and dry weather. It was possible to generate a cool supply of air at satisfactory conditions using solar energy only for all clear days under the local weather conditions.     

Periodic, fully developed, natural convection in a channel with corrugated confining walls

Numerical solutions are obtained for natural convection heat transfer in an open channel with corrugated, isothermal confining walls. The channel is very long so that the fluid temperature approaches the wall temperature and the flow can be assumed to be periodically fully developed. The solutions are obtained by solving the full elliptic governing equations in a transformed coordinate system which maps the channel with corrugated walls onto a channel with flat walls. The periodic, fully developed Nusselt number for the corrugated channel is expressed by the relation Nu = CGrPr/(L/W) where Gr, Pr and L/W are the Grashof number, the Prandtl number and the aspect ratio, respectively, and C is a parameter which is a function of Gr, L/W and the corrugation angle θ. In the limiting case of θ = 0° (two flat walls), the parameter C approaches a constant value. This value is within 1.6% of the exact analytical result.     

Experimental study of interzonal natural convection through an aperture

The work was concerned with measuring natural convection through an aperture between two zones in an environmental chamber. Airflow rates between the two zones were measured using a tracer-gas decay technique, and the temperature at the centre of each zone was measured using thermocouples. Zone 1 was heated to various temperatures in the range 18–38°C using thermostatically controlled heaters. Zone 2 was unheated. A multipoint sampling unit was used to collect a tracer-gas sample from each zone. The concentration of SF₆ tracer was measured using an infra-red gas analyzer. The heat and mass flow rates between the two zones were calculated from the tracer-gas concentrations and temperature differences. Results were compared with values predicted by existing algorithms for two-zone enclosures. The mass flow rate through the aperture was found to be a function of the temperature difference between the two zones.     

Boiling Heat Transfer Performance in a Spiraling Radial Inflow Microchannel Cold Plate

ABSTRACT

This study presents an experimental exploration of flow boiling heat transfer in a spiraling radial inflow microchannel heat sink. The effect of surface wettability, fluid subcooling, and mass fluxes are considered. The design of the heat sink provides an inward radial swirl flow between parallel, coaxial disks that form a microchannel of 300 microns. The channel is heated on one side, while the opposite side is essentially adiabatic to simulate a heat sink scenario for electronics cooling. To explore the effects of varying surface wetting, experiments were conducted with two different heated surfaces. One was a clean, machined copper surface and the other was a surface coated with zinc oxide nanostructures that are superhydrophilic. During boiling, increased wettability resulted in quicker rewetting and smaller bubble departure diameter, as indicated by reduced temperature oscillations during boiling, and achieving higher maximum heat flux without dryout. The highest heat transfer coefficients were seen in fully developed boiling with low subcooling levels as a result of heat transfer being dominated by nucleate boiling. The highest heat fluxes achieved were during partial subcooled flow boiling at 300 W/cm² with an average surface temperature of 134° Celsius. Recommendations for electronics cooling applications are also discussed.     

NATURAL CONVECTION IN A VERTICAL,ASYMMETRICALLY HEATED,PERMEABLE WALLED CHANNEL

Developing natural convection in an asymmetrically heated, open-ended vertical channel was studied both experimentally and numerically. A tightly stretched, perforated, plastic radiation shield was suspended parallel to an electrically heated aluminum plate to form a vertical channel. In order to model the heat transfer and fluid flow in the vertical channel, the unsteady, two-dimensional Navier-Stokes equations were solved using a primitive variable, finite-difference formulation. Flow through the perforated boundary was modeled using a modified form of Darcy's law. Radiative exchange between the boundaries and between the boundaries and the environment was included. The predicted mass flow was within 3% of that measured experimentally. Both the average plate temperature and the bulk exit channel air temperature were within 1·2°C of the measured values. However, the predicted average temperature of the radiation shield was 8°C higher than that measured.     

Natural convection in enclosures with floor cooling subjected to a heated vertical wall

Natural convection in enclosures is extensively investigated due to its importance in many applications, such as heat transfer through double glazing windows, electronic cooling devices, geophysical applications, etc. Two configurations that have been extensively explored in the literature are the differentially heated enclosures and the Rayleigh–Benard problems. In the present work, a different kind of problem is investigated, namely the cross thermal boundary conditions. Three dimensional analyses were performed for an enclosure cooled from below with one vertical wall heated, and the other connecting walls were assumed to be adiabatic. The thermal condition at the ceiling is varied from an adiabatic one to a different degree of heating. The objective of this study is to simulate the comfort provided by floor cooling in a room. For comfort requirements, the interest is on determining the rate of heat transfer and the temperature distribution in the room. Also, the results have importance for other cooling applications such as electronic cooling and natural convection in freezers. Furthermore, the problem is academically interesting for understanding the fundamentals of natural convection. Based on the authors’ knowledge, the physics of this problem has not been explored by other people in such a detail. However, the application has been in practice from ancient times.The predicted results are interesting and have practical applications. For a certain configuration, where strong three dimensional recirculations were predicted, the flow is three dimensional, hence the two dimensional assumption is not valid. Also, it is found that the rate of rate transfer from the floor is a weak function of the investigated parameters, such as Rayleigh number.     

Simulation of solar air heating at constant temperature

Solar space heating with warm air in typical air collectors and rock bed storage systems involves constant air flow rates and varying the temperature of supply to rooms and to storage. This practice results in undesirable fluctuations in comfort levels in the living space, excessive storage size, useful but inaccessible heat in storage, and unnecessarily high energy consumption for air circulation and auxiliary heat. These drawbacks can be avoided by use of a practical controller and variable speed fan to provide heated air from the collector at constant temperature and a continually varying flow rate. Collector manufacturer's data, confirmed by seasonal tests on a solar air heating system in Solar House II at Colorado State University, have been used in simulations at constant hot air supply temperatures of 40°, 50°, and 60°C, and at one typical constant flow rate of 49 kg/h per m² through a 50 m² collector and rock bed storage unit, providing approximately half the seasonal heating requirements of a residential building. Auxiliary heat requirements and fan power use in the 40°C and 50°C constant temperature operations were significantly reduced from the levels prevailing under constant flow conditions. Collection efficiency and solar heat supply at constant flow were slightly higher than values at the 60°C constant temperature level.     

Chimney effect in a “T” form cavity with heated isothermal blocks: The blocks height effect

In this paper, a numerical study of natural convection from a two dimensional “T” form cavity with rectangular heated blocks is conducted. The blocks are identical, and the domain presents a symmetry with respect to a vertical axis passing through the middle of the opening. The governing equations are solved using a control volume method, and the SIMPLER algorithm for the velocity–pressure coupling is employed. Special emphasis is given to detail the effect of Rayleigh number and block height on the heat transfer and the flow rate generated by the chimney effect. The results are given for the parameters of control as, 10⁴Ra3×10⁶, Pr=0.71, opening diameter (C=l^′/H^′=0.15), blocks gap (D=d^′/H^′=0.5) and blocks height (1/8B=h^′/H^′1/2). These results show that the heat transfer variation with Ra is in the same manner as those met in the case of the vertical smooth or ribbed channels.     

竖直矩形窄通道内饱和沸腾起始点的研究

以去离子水为工质,对尺寸为720 mm×250 mm×3.5 mm的单面电加热竖直矩形窄通道内饱和沸腾起始点进行实验研究。分析了加热热流密度、工质进口温度和质量流量对饱和沸腾起始点位置及饱和沸腾起始点处壁面过热度的影响。在已有饱和沸腾起始点预测关联式的基础上,对实验数据进行非线性回归分析,得到适用于单面加热矩形窄通道饱和沸腾起始点的新关联式。结果表明：新拟合的关联式预测值与实验值的平均相对误差为17.63%,能很好的预测常压、低加热热流密度与低流速条件下的饱和沸腾起始点处壁面过热度与热流密度的关系。     

Hybrid ventilation in two interconnected rooms with a buoyancy source

The design of energy efficient buildings and the potential for using solar energy for heating and cooling is contingent upon optimizing the building ventilation systems. In this paper, we study the ventilation of two interconnected spaces, such as adjacent offices or areas in an open plan office. The goal is to locate return vents to increase the efficiency of night ventilation and to reduce energy consumption.The flow in two interconnected rooms of similar sizes is studied experimentally using a tank divided by an interior vertical wall. A forced buoyancy source with a finite volume flux is located in the ceiling of one-room and an unforced vent is opened in the ceiling of the other room. The goal of the study is to understand the transient cooling/heating that occurs in this two-room system when a forced cold-air vent is located in the ceiling of the first room and a return ventilation exit is located in the second. In particular, we investigate the effects of varying the number of openings and their vertical positions in the interconnecting wall. First, a single opening at the bottom, middle or top of the shared wall is examined. Second, the case of two openings in the wall is considered, with the openings located at the top–bottom, top–middle, bottom–middle, and finally at two mid locations in the wall. The results are compared with the one-room case, which represents the reference case.It was found that, irrespective of the number and locations of the openings, the flow evolves into a quasi-stationary stably stratified two-layer system, with the depths of the layers being different in each room. The average temperature inside each room initially decreases linearly with time and approaches the supply-air temperature at large times. This initial linear decrease holds until cold-air leaves the unforced room through the top-vent at time t_e. Subsequently, temperature decreases as an exponential function of time with a characteristic filling time τ = V/Q_s, where V is the total volume of both rooms and Q_s is the source volume flux. The efficiency of the ventilation depends on the time t_e, and this depends, in turn, on an exchange flow that is established between the two-rooms by the differences in density in each room. For a single opening, the exchange flow takes place as a two-way flow in the opening, while for two openings the flow is from the forced room through the lower opening and in the opposite direction through the upper opening.When the upper opening is located below the ceiling, this flow from the unforced room ‘shields’ the return vent from the dense fluid, thereby increasing the efficiency of the ventilation.     

Experimental study of impingement spray cooling for high power devices

An experimental study of a closed-loop impingement spray cooling system to cool a 1 kW 6U electronic test card has been conducted. The system uses R134a as working fluid in a modified refrigeration cycle. The spray from four vapor assisted nozzles is arranged to cover a large ratio of the heated area of the card. Investigations are currently focused on effects of mass flow rate, nozzle inlet pressure and spray chamber pressure. Experimental results are promising with a stable average temperature of around 23 °C being maintained at the heated surface, and maximum temperature variation of about 2 °C under suitable operating conditions. Heat transfer coefficients up to 5596 W/m² K can be achieved with heat flux input around 50,000 W/m² in this study. It is found that cooling performance improved with increasing mass flow rate, nozzle inlet pressure and spray chamber pressure, whereas uniformity of the heated surface temperature can only be improved with higher mass flow rate and nozzle inlet pressure. The mechanisms for the enhanced performance are also presented.     

Preliminary experimental study of a bio-inspired,phase-change particle capillary heat exchanger

In this study a new cooling concept using encapsulated phase-change particles flowing with water in a parallel-plate mini-channel is presented. This novel concept is inspired by the gas exchange process in alveolar capillaries, where red blood cells (RBCs) flow with blood plasma, yielding very high gas transfer efficiency. Another important characteristic of alveolar capillary blood flow, which is related to the high efficiency of the lungs, is the snug fitting of the RBCs into the capillaries. Hence, preliminary results of experimental tests using particles with diameter similar to the flow channel spacing flowing with water through a heated parallel-plate channel test module are presented and analyzed. The particles are octadecane paraffin (C₁₈H₃₈), a phase-change material, encapsulated in a thin melamine shell. The temperature distribution along the heated surface of the channel is measured for various water flow rates, with and without particles, and with different number of particles. Results are reported in terms of the channel heated surface average temperature and the average heat transfer coefficient, showing a sensible increase (over 20%) in the latter as compared to a clear (of particles) flow. There is strong evidence the increase in heat transfer efficiency to result from a combination of the extra mixing flow effect caused by the presence of particles in the flow and the phase-change effect caused by the EPCM inside the particles.     

Thermal design of a roof as an inexpensive solar collector/storage system

This paper presents the thermal performance of a roof as a solar collector/storage system which is important for the thermal design of buildings. The system consists of a mass of concrete or concrete insulation, one face of which is blackened/glazed and exposed to solar radiation and ambient air, while the other is in contact with room air at constant temperature. The heat can be extracted by the passage of water through the network of tubes in this block. It is seen that, by increasing the depth of the tubes, the rise in water temperature decreases but the time difference between the maxima of the solair temperature and that of the outlet water temperature increases. At a tube depth of 0·10 m, the maximum temperature rise of the water is 33·5°C. The corresponding efficiency of the system is 28·0% while the flow rate of water is 5·0 litre/h m²; the heat flux entering the room is also reduced considerably.     

PMMA-assisted synthesis of Li1−xNi0.5Mn1.5O4−δ for high-voltage lithium batteries with expanded rate capability at high cycling temperatures

In this work, poly(methyl methacrylate) (PMMA), a non-surfactant polymer was used to synthesize nonstoichiometric Li_0.82Ni_0.52Mn_1.52O_4−δ (0 ≤ δ ≤ 0.25) spinels. The presence of the polymer was found to be beneficial with a view to facilitating the use of the spinel in electrodes for lithium batteries. Thus, PMMA allowed spinel particles of a high crystallinity and uniform size and shape to be obtained, and particle size to be tailored by using an appropriate calcining temperature and time. By controlling these variables, spinels in nanometric, submicrometric and micrometric particle sizes were prepared and characterized by chemical analysis, X-ray diffraction, electron microscopy, thermogravimetry and nitrogen adsorptions measurements. The spinels were obtained as highly crystalline phases with lithium and oxygen deficiency and some cation disorder as revealed by chemical analysis, thermogravimetric and XRD data. Their electrochemical performance in two-electrode cells was tested at room temperature and 50 °C over a wide range of charge/discharge rates (from C/4 to 4C). Cell performance was found to depend on particle size rather than on structural properties. Thus, the spinel best performing at 50 °C was that consisting of submicrometric particles, which delivered a high capacity and exhibited the best capacity retention and rate capability. Particles of submicronic size share the advantages of nanometric particles (viz. the ability to withstand high charge/discharge rates) and micrometric particles (a high capacity and stability at low rates).     

Laminar natural convection in inclined open shallow cavities

Laminar steady state natural convection in inclined shallow cavities has been numerically studied. The side facing the opening is heated by a constant heat flux, sides perpendicular to the heated side are insulated and the opening is in contact with a fluid at constant temperature and pressure. Equations of mass, momentum and energy are solved using constant properties and Boussinesq approximation and assuming an approximate boundary conditions at the opening. Isotherms and streamlines are produced, heat and mass transfer is calculated for Rayleigh numbers from 10³ to 10¹⁰, cavity aspect ratio A=H/L from 1 to 0.125. The results show that flow and heat transfer are governed by Rayleigh number, aspect ratio and the inclination. Heat transfer approaches asymptotic values at Rayleigh numbers independent of the aspect ratio. The asymptotic values are close to that for a flat plate with constant heat flux. The effect of elongation of open cavities is to delay this asymptotic behavior. It is also found that the inclination angle of the heated plate is an important parameter affecting volumetric flow rate and the heat transfer.     

Static heat transfer to liquid helium in open pools and narrow channels

The transfer of heat to boiling liquid helium has been measured in open pools and narrow channels. In open pools a marked dependence of heat transfer on the orientation of the heated surface is observed. The maximum heat flux for nucleate boiling varies from 1 W/cm² with the heated surface horizontal facing upwards to about 0·1 W/cm² with the surface horizontal facing downwards. In a narrow vertical channel the maximum heat flux is reduced to about 0·15 W/cm² for a rectangular channel 10mm × 1 mm (50 cm length), and appears to decrease linearly with the channel dimension. The heat transfer is considerably increased in the narrow channel when the fluid is pressurized.     

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.     

Thermal contact resistance between plasma-sprayed particles and flat surfaces

Plasma-sprayed molybdenum and yttria-stabilized zirconia particles (38–63 μm diameters) were sprayed onto glass and Inconel 625 held at either room temperature or 400 °C. Samples of Inconel 625 were also preheated for 3 h, and then air-cooled to room temperature before spraying. Photographs of the splats were captured by using a fast charge-coupled device (CCD) camera. A rapid two-color pyrometer was used to collect thermal radiation from the particles during flight and spreading to follow the evolution of their temperature. The temperature evolution was used to determine the cooling rate of spreading particles. An analytical heat conduction model was developed to calculate the thermal contact resistance at the interface of the plasma-sprayed particles and the surfaces from splat cooling rates. The analysis showed that thermal contact resistance between the heated or preheated surfaces and the splats was more than an order of magnitude smaller than that on non-heated surfaces held at room temperature. Particles impacting on the heated or preheated surfaces had cooling rates that were significantly larger than those on surfaces held at room temperature, which was attributed to smaller thermal contact resistance.     

西门子公司V94.3燃气轮机冷却空气信息推测

作为建立燃用低热值合成气的燃气轮机变工况模型的一个关键步骤，对西门子V94.3燃气轮机冷却空气参数及其分配进行了研究，试图从公开发表的燃气轮机功率、压比、排气温度、三亿透平初温等数据中推测出冷却空气量的分配规律。计算和推测所得到的冷却空气参数和分配规律与燃机净功率以及ISO温度基本吻合。