The Performance of Porous Electric Heaters

The performance of a proposed porous electric heater is investigated. The porous heater exchanges heat with the working fluid through its large volumetric surface area. As a result, it produces lower surface temperature as compared with the conventional heater for the same imposed heating power. Two mathematical models are presented to describe the thermal behavior of both heaters and the predictions of both models are compared at different operating conditions.     

Studies on thermal performance of electrical heaters by using porous materials

This paper is devoted to the subject of new configurations of electrical heaters that use porous media. Thermal performance of two innovative designs are estimated and compared with that of the conventional ones. In the first design, airway is filled with a porous insert while the second heater uses a porous heating element instead of the conventional solid ones. The coupled differential equations for both solid and gas phase are solved simultaneously by a control-volume based procedure along with the corresponding boundary conditions, using SIMPLE algorithm. Results reveal that for equal air flow rates, equal heater masses, and the same energy generation rates, the heating element in the porous heater attains the lowest temperature. Surprisingly, this heater also achieves the lowest pressure drop, which is a result of lower air velocity inside the heater. The results also show that at the same aforementioned conditions, the heater with porous insert suffers from the highest temperature while it has a medium pressure drop with respect to the other designs.     

Long-term thermal performance of a two-phase thermosyphon solar water heater

This article investigates experimentally the long-term thermal performance of a two-phase thermosyphon solar water heater and compares the results with the conventional systems. Experimental investigations are conducted to obtain the system thermal efficiencies from the hourly, daily and long-term performance tests. Different heat transfer mechanisms, including natural convection, geyser boiling, nucleate boiling and film-wise condensation, are observed in the two-phase thermosyphon solar water heater while solar radiation varies. The thermal performance of the proposed system is compared with that of four conventional solar water heaters. Results show that the proposed system achieves system characteristic efficiency 18% higher than that of the conventional systems by reducing heat loss for the two-phase thermosyphon solar water heater.     

Analytical and experimental studies on the thermal performance of cross-corrugated and flat-plate solar air heaters

The cross-corrugated heaters consist of a wavelike absorbing plate and a wavelike bottom plate, which are crosswise positioned to form the air flow channel. Two types of these heaters are considered. For the type 1 heater, the wavelike shape of the absorbing plate is along the flow direction and that of the bottom plate is perpendicular to the flow direction, while for the type 2 heater it is the wavelike shape of the bottom plate that is along the flow direction and that of the absorbing plate is perpendicular to the flow direction. The aims of the use of the cross-corrugated absorbing plate and bottom plate is to enhance the turbulence and the heat-transfer rate inside the air flow channel, which are crucial to the improvement of efficiencies of solar air-heaters. To quantify the achievable improvements with the cross-corrugated absorbing and bottom plates, flat-plate solar air-heaters which have both a flat absorbing plate and a flat bottom plate, are also considered. The thermal performance of these three types of solar air-heaters are analyzed, measured and compared under several configurations and operating conditions. All the analytical and experimental results show that, although the thermal performance of the type 2 heater is just slightly superior to that of the type 1 heater, both of these cross-corrugated solar air-heaters have a much superior thermal performances to that of the flat-plate one. It is also found that the use of selected coatings on the absorbing plates of all the heaters considered can substantially enhance the thermal performances of the heaters, whereas such a selected coating on the bottom plates or/and the glass covers does not have such a significant effect on the thermal performances of the heaters.     

MIXED CONVECTION IN A CHANNEL WITH POROUS MULTIBLOCKS UNDER IMPOSED THERMAL MODULATION

This study is made of an enhancement of a mixed-convection heat transfer in a channel containing multiple porous blocks heated from below. The heat flux from the most upstream heater varies in a sinusoidal form, while other heaters have a constant heat flux. The Brinkman-Forchheimer-extended Darcy model and two-equation energy model are adopted to characterize the flow and temperature fields inside porous regions. The explicit effect of thermal modulation at the upstream heater is examined by acquiring comprehensive numerical solutions. The heat transfer enhancement is pronounced at the far downstream heaters when resonance is realized. The resonance frequency is close to the characteristic frequency of the system, which scales with the time for the main stream to travel from a heater to a neighboring heater. The evolutions of flow and temperature fields are exemplified to provide physical interpretations. The effects of pore density and of porous block height are reported. The benefit of heat transfer augmentation, as opposed to the increased friction factor, is assessed to justify the use of thermal modulation in the upstream heater.     

An optimum performance of flat-type solar air heater with porous absorber

This investigation is concerned with the design and performance of a flat-type solar air heater in which air flows perpendicularly from the transparent cover to a porous absorber plate. The design phase involved a stability analysis to determine the critical distance (maximum allowable distance) between the absober and transparent cover, for suppressing convection currents, at various environmental and operating conditions. These results are useful to designers of solar collectors of the proposed type. In addition, the thermal performance of this solar heater at its optimum design conditions was computed for a wide range of system parameters illustrating the contribution of conduction and radiative modes of heat transfer. The results indicate that the best operating efficiency can be obtained when running the collector with a mass flow rate of m > 40 kg/m².h. Furthermore, the collector thermal performance is superior than channel type solar air heaters operating under similar conditions and much simpler than honeycomb porous bed solar air heaters.     

Thermal and thermo-hydraulic performance investigation of double-pass packed bed solar air heaters under external recycle

In the present investigation, two types (Type A and Type B) of the double-pass packed bed solar air heater under external recycle are investigated theoretically. In Type A, the porous media is considered in the upper channel, whereas in Type B, the porous media is considered in the lower channel. Iron scraps are used as a packed bed material (porous media) to strengthen the convective heat transfer coefficient for air flowing through the packed bed. The mathematical model for these two air heaters operating under forced convection mode is presented. The results revealed that the thermal and thermo-hydraulic efficiencies of Type A are higher as compared to Type B. In order to validate the models, the theoretical results obtained from the conventional model of Type B are compared with the theoretical results obtained from the previous investigation and showed that good agreement is achieved.     

Energy efficiency and indoor thermal perception: a comparative study between radiant panel and portable convective heaters

This study investigates experimentally the thermal perception of indoor environment for evaluating the ability of radiant panel heaters to produce thermal comfort for space occupants as well as the energy consumption in comparison with conventional portable natural convective heaters. The thermal perception results show that, compared with conventional convection heater, a radiantly heated office room maintains a lower ambient air temperature while providing equal levels of thermal perception on the thermal dummy head as the convective heater and saves up to 39.1% of the energy consumption per day. However, for human subjects’ vote experiments, the results show that for an environmentally controlled test room at outdoor environment temperatures of 0°C and 5°C, using two radiant panel heaters with a total capacity of 580 W leads to a better comfort sensation than the conventional portable natural convective heater with a 670 W capacity, with an energy saving of about 13.4%. In addition, for an outdoor environment temperature of 10°C, using one radiant panel heater with a capacity of 290 W leads to a better comfort sensation than the conventional convection heater with a 670 W capacity, with an energy saving of about 56.7%. From the analytical results, it is found that distributing the radiant panel heater inside the office room, one on the wall facing the window and the other on the wall close to the window, provides the best operative temperature distribution within the room.     

Thermal performance assessment of recyclic double-pass flat and V-corrugated plate solar air heaters

Double-pass solar air heaters occupy an important place among solar air heating systems, because of minimal heat loss and maximum thermal efficiency with marginal heater size and cost. In the present work, investigations related to the thermal performance predictions have been carried out for double-pass flat and V-corrugated absorber plate solar air heaters under recycle operation. The mathematical models proposed herein are solved using an analytical approach that uses an iterative solution procedure. Furthermore, based on simulation results obtained from the analytical study, the optimum value of the recycle ratio, the mass flow rate, the absorptivity and the emissivity at which the heaters yield the maximum value of the thermal efficiency have been identified and presented using response surface methodology (RSM). The results of RSM revealed that the mathematical models are significant. In addition, results of the present study are validated and compared with previous studies. A reasonable agreement and significant improvement have been achieved.     

Thermohydraulic performance of solar air heaters with roughened absorber plates

Artificial roughness has been found to enhance the heat transfer from the absorber plate to the air in a solar air heater duct. However, this improvement is invariably accompanied by increased pumping power. In this work, the effect of roughness and operating parameters on the thermal as well as the hydraulic performance of roughened solar air heaters is discussed and the thermohydraulic performance of roughened solar air heaters is compared with that of conventional smooth solar air heaters. The optimum design and operating conditions have been determined. On the basis of thermohydraulic considerations it has been found that the systems operating in a specified range of Reynolds number show better thermohydraulic performance depending upon the insolation. A relationship between the system and operating parameters that combine to yield optimum performance has been developed.     

A review of the performance of double pass solar air heater

Improvement of the thermal performance of a solar air heater can be obtained by enhancing the rate of heat transfer. The thermal efficiency of double pass solar air heater is higher in comparison to single pass with the concept involved of doubling the heat transfer area without increase in the system cost. Numbers of studies have been carried out on the performance analysis of double pass solar air heater provided with heat transfer augmentation techniques viz. using extended surfaces, packed bed, corrugated absorber were reported in the literature and found more increase in the thermal efficiency in comparison to conventional double duct solar air heater. These studies includes the design of double pass solar air heater, heat transfer enhancement, flow phenomenon and pressure drop in duct. This paper presents an extensive study of the research carried out on double pass solar air heater. Based on the literature review, it is concluded that most of the studies carried out on double pass solar air heater integrated with porous media and extended surfaces. Few studies were carried out with corrugated absorber. Further no study has been reported so far on double pass solar air heater with absorber plate artificially roughened from both the sides. Mathematical models based on energy analysis of some configurations of solar air heater have been discussed.     

热泵热水器技术及应用

热泵热水器借助外部能量能把不能直接利用的低位热源转换为可以利用的高位热能,从而达到节约部分高位能的目的。阐述了热泵热水器的发展历史与应用现状,分析了空气源热泵的工作原理和节能原理,并将空气源热泵热水机组与电热水器、燃油热水锅炉、燃气热水锅炉等热水供应系统的性能进行了比较。在此基础上,结合企业工程应用实例,指出了推广热泵技术对于节能减排的意义。     

Comparative study of three thermosyphon solar water heaters made of flat-plate collectors with different absorber configurations

The paper presents a comparative study of three solar water heaters made of flat-plate collectors with different absorber configurations. The performance of the three solar water heaters is assessed under the same conditions. The collectors have the same surface area and are glazed. The theoretical model for each collector type, with the transient effects taken into account, is based on a control volume. By considering a small element of the collector in each case and the storage tank, six partial differential equations were developed for each solar water heater and were solved numerically for a cloudy day. This study shows that the thermal performance obtained with the solar water heater using the absorber-pipe lower bond configuration in the solar collector is always greater than the two others. These results showed that the solar water heater made of the absorber-pipe lower bond configuration is more efficient than the other systems.     

Theoretical and experimental investigations of a two-phase thermosyphon solar water heater

This article experimentally and theoretically investigates a two-phase thermosyphon solar water heater. The performance of this innovative solar water heater at different solar radiation intensities and tilt angles are experimentally discussed. The results show the best charge efficiency of the system is 82%, which is higher than the conventional solar water heaters. The theoretical model is also developed using the thermal resistance-capacitor method. The simulation predictions agree well with the experimental data within an average error deviation of ±6%. Two methods for improving the performance of this heater, double fin tubes and nano particle, are proposed. The results show that charge efficiencies can increase 3% and 4%, respectively, according to the theoretical model.     

提高热电厂效率的几项措施

从热经济性角度提出提高热电厂效率的几项技术和措施:通过凝汽器补充软化水,将外供蒸汽过热度降低;使用喷射式混合加热器回收热力除氧器排汽,作为生水加热器;利用压力匹配器代替减压减温器;用两相流加热器代替面式高压加热器等。     

Transient behaviour of collector/storage solar water heaters for generalised demand patterns

This communication presents a simple transient model for predicting the thermal performance of collector/storage solar water heaters for generalised demand patterns. These heaters consist of either (i) an insulated rectangular metallic tank whose top surface is blackened and suitably glazed (i.e. a built-in storage solar water heater) or (ii) an insulated open shallow tank with black bottom.inner sides and a glass plate at the surface in contact with the water (i.e. a shallow solar pond water heater). The time dependence of the water temperature for the withdrawal of hot water from the system at constant flow rates constantly or intermittently has been explicitly evaluated. Numerical results for the operation of the system in industrial and community service applications are discussed.     

Influence of Thermophysical Wall Properties During Pool Boiling on Large Diamond and SiC Heaters

The results of pool boiling experiments with synthetic diamond and silicon carbide (SiC) heaters are presented for water as the boiling liquid. The diamond and SiC heaters varied considerably in thermal conductivity, but they had smooth, nearly identical surfaces, which was also the case in regard to their contact angles for water. Temperature sensors and electric heating wires were directly vapor-deposited underneath the surfaces. The experiments were carried out with comparable large heaters (15 mm × 15 mm) for pure water under atmospheric pressure (1 bar) in a pool boiling cell. The heat transfer characteristics including the corresponding boiling curves were obtained. In prior work, it was found that the influence of the thermophysical wall properties might be substantial in the case of special heater geometries, leading to trapped bubbles, but no significant differences between both materials were observed in the case of conventional heater configurations.     

Investigation of thermal performance of-double pass-flat and v-corrugated plate solar air heaters

In this paper, the double pass flat and v-corrugated plate solar air heaters are investigated theoretically and experimentally. Analytical models for the air heater with flat and v-corrugated plates are presented. Numerical calculations have been performed under Tanta (latitude, 30^° 47^′ N) prevailing weather conditions. The theoretical predictions indicated that the agreement with the measured performance is fairly good. Comparisons between the measured outlet temperatures of flowing air, output power and overall heat losses of the flat and v-corrugated plate solar air heaters are also presented. The effect of mass flow rates of air on pressure drop, thermal and thermo hydraulic efficiencies of the flat and v-corrugated plate solar air heaters are also investigated. The results showed that the double pass v-corrugated plate solar air heater is 11-14% more efficient compared to the double pass flat plate solar air heater. It is also indicated that the peak values of the thermo hydraulic efficiencies of the flat and v-corrugated plate solar air heaters are obtained when the mass flow rate of the flowing air is 0.02 kg/s.     

Performance and Service Life Considerations—Closed Feedwater Heaters

Because of the potential restriction of plant capacity and increases in fuel consumption cost that result from the removal of closed feedwater heaters from service, there is a need for the application of new design and operation concepts that enhance both the thermal performance and the availability of heaters and their associated systems. Practical operating practices, which take into consideration the highly localized environments in closed feedwater heaters and the individual susceptibilities to failure of the various tube materials in common use, have evolved from direct experience with closed feedwater heater problems. Many electric utilities have adapted these operating concepts in their standard operating procedure. Life extension programs for feedwater heaters that are repaired or to be replaced must include the provision for new operating concepts. Heaters that are subjected to the same operation conditions that led to the original failures may be expected to fail for the same reasons, often in a much shorter time. The acquisition of accurate data on operating performance, the location of tube distress, combined with the application of new, successful practices in feedwater heater design, operation, and maintenance, have provided the best assurance of a satisfactory service life of closed feedwater heaters.     

大型火电机组加热器动态学模型的研究

以在大型火电机组中广泛使用的三段式加热器为研究对象，充分考虑了加热器结构参数的工质物性参数对传热过程的影响以及静态计算与动态教学模型间的差别，建立了加热器动态数学模型。仿真实验与工程应用证明该模型具有较高的静态计算精度和良好的动态响应特性，具有较广泛适用性和一定的工程应用价值。