Experimental thermal storage research of organic binary phase change materials in building environment

This paper aims to reveal the heat transfer mechanism of low-temperature phase change material (PCM) and design PCM heat storage device in building heating environment. Firstly, low-temperature binary PCMs of lauric acid and stearic acid are prepared, and their thermal properties are investigated by DSC. Then, shell and tube latent heat thermal energy storage units are conducted, and heat transfer experiments are carried out to analyze the heat transfer mechanism of PCM. The results demonstrate that natural convection plays an important role in heat transfer process, and the heat storage efficiency of PCMs can be significantly enhanced by increasing the fin width and improving the inlet heat transfer fluid (HTF) temperature. Furthermore, some proposals are put forward to guide the design of PCM storage device in building heating environment.     

Experimental energy and exergy analyses of a discharging heat exchanger for a small hot-oil domestic storage tank

Experiments are described to investigate the thermal performance of a discharging heat exchanger for a small storage tank filled with oil. Experimental results are presented in terms of the discharging energy rates (power) and the discharging exergy rates for low (~4 ml/s) and high discharging flow rates (~8 ml/s). Water heating energy rates, which are respectively maximized at approximately 600 W and 1200 W at low and high flow-rate discharging, are found to be higher than the discharging energy rates, which are respectively maximized at 450 W and 900 W. These results indicate that the energy rates do not accurately evaluate the thermal performance of the discharging heat exchanger since the energy heating rate of the water is greater than that for the oil that heats it, which is thermodynamically inconsistent. The energy rates should thus be used with caution when the thermal performance of the heat exchanger is evaluated. Water heating exergy rates, which are respectively maximized at approximately 45 W and 130 W at low and high flow-rate discharging, are generally smaller than the discharging exergy rates, which are respectively maximized at 65 W and 170 W. Exergy rate results are thus more consistent in the physical process of water heating, and an exergy factor is suggested as a proper measure for evaluating the performance of the discharging heat exchanger. The maximum value of the exergy factor is found to increase from 0.15 at low flow rates to a maximum value of approximately 0.19 at high flow rates. This implies that to extract more energy from a storage tank to a discharging heat exchanger, the flow rate has to be high, which is consistent with the physical process of heating water faster to higher temperatures. The exergy factor can thus be used as a design parameter for discharging heat exchangers.     

Experimental and numerical analysis of a helical coil solar cavity receiver: Thermal oil as the heat transfer fluid

The paper presents the novel design of double glazing helical coil solar cavity receiver for solar thermal applications. Performance model has been developed for the experimental setup based on energy balance equations. The results obtained were compared with horizontal tube receiver for the same experimental setup. The result shows that the 87.96% improvement in the convective heat transfer coefficient for the double glazing helical coil solar cavity receiver. Maximum conversion efficiency achieved is 21% more than that would be obtained for horizontal tube receiver. This paper also investigates how the quality of vacuum degraded with the temperature of the glass cover.     

Performance Model for a Horizontal Tube Falling Film Evaporator

A model is developed to predict the evaporative heat transfer coefficient in a horizontal tube-falling evaporator and has been applied to evaluate overall performance of a desalination unit. Performance variation with different parameters like operation temperature, type of water distribution system, mass flow rate of distilled water inside the exchanger are analyzed. It has been observed that the model is able to predict the trends of heat transfer characteristics of the evaporator reasonably well. However, at low liquid film flow rate conditions, the model overpredicts the heat transfer characteristics marginally. In order to improve the evaporative exchanger performance, it is observed that preheating of the liquid film before injection into the evaporator is desirable. Calculations are also performed to estimate the value of overall heat transfer coefficient for a typical desalination unit.     

Experimental investigation on the phase change material-based modular heat exchanger for thermal management of a building

A phase change material (PCM)-based flat plate modular heat exchanger for free cooling application, suitable for the diurnal temperature variation that prevails during the summer months of Bangalore city, India is designed and experimentally investigated. The flow and other parameters selected in the present study are meant to suit the accelerated charging of the PCM in the modular heat exchanger during the early morning hours, and to provide cool energy to the room during the daytime, by circulating the ambient air through the heat exchanger at a lower velocity. It is observed from the charging experiments that the decrease in the inlet air temperature has more influence in reducing the solidification time than the increase in the inlet air velocity. The heat exchanger designed in the present investigation is capable of maintaining the room temperature around 30°C for a longer duration of 8 hr when the heat load is 0.5 kW. It is suggested to design the modular heat exchanger with a surface area proportionate to the present heat exchanger size when the room heat load increases beyond 0.5 kW, in order to maintain a minimum comfortable temperature of 30°C in the room.     

Thermodynamic coupling characteristics in hybrid (dry/wet) cooling system

A hybrid cooling system consisting of both a dry section and wet section is proposed in this paper as a means to conserving energy and water by combining the benefits of both dry and wet cooling modes. A new thermodynamic coupling characteristics computing model was established to identify the best combination of dry and wet cooling subsystems in the hybrid tower throughout year-round operation based on its air thermodynamic state under the “no plume” principle. A hybrid cooling tower in Inner Mongolia, China, consisting of an elliptical tube heat exchanger with rectangular fins and counter-flow wet packing, was designed as an example under the no plume principle. The minimum number of heat exchanger units in service and the corresponding thermodynamic operating parameters were obtained under a year-round operation. The tower exhibited notable advantages in regards to water conservation compared to the traditional evaporative cooling tower at an estimated yearly savings of 3.74 × 10⁷ kg water.     

Numerical parametric study of a new earth-air heat exchanger configuration designed for hot and arid climates

ABSTRACT

Thermal potential for cooling and heating can be achieved by new configuration of earth–air heat exchanger (EAHE). This paper presents a numerical investigation of thermal performance of a spiral-shaped configuration of EAHE intended for the summer cooling in hot and arid regions of Algeria. A commercial finite volume software (ANSYS FLUENT) has been used to carry out the transient three-dimensional simulations and the obtained results have been validated using the experimental and numerical data obtained from the literature. The agreement between our simulation results and those from literature is very satisfactory. A parametric analysis of the new geometry of (EAHE) has been performed to investigate the effect of pitch, depth, pipe length and of the flow velocity on the outlet air temperature and the EAHE’s mean efficiency as well as its coefficient of performance (COP). It has been shown that when the pitch space varies between 0.2 and 2 m the difference of outlet air temperature increases by 6 ^°C. When the air velocity increases from 2 to 5 m/s the mean efficiency decreases from 60 % to 33 % and the COP of the EAHE decreases from 2.84 to 0.46.     

Experimental investigation on convection heat transfer characteristics of flat plate under environmental wind condition

By establishing wind tunnel and employing electrical heating method, the heat transfer characteristics of flat plate were investigated under environmental wind condition. Both the uniform and linear heat flux boundary conditions were adopted for comparison purpose. Besides, the impacts of heat flux q_wm, tilt angle α and wind incidence angle θ on heat transfer were explored in cases of windward and leeward facing positions. The local convection heat transfer coefficient h_cx and average convection Nusselt number Nu_cm were obtained. The results show that, when α is small, there are two maximum values of h_cx under linear heat flux boundary condition. As for Nu_cm, differences between the two boundary conditions seem indistinctive. At windward orientation, Nu_cm is not sensitive to α. While for leeward orientation, Nu_cm has a rapid decline progress with the increasing α, which indicates the tilt angle α is an important parameter to affect the heat transfer of plate. Finally, new correlations of Nu_cm have been developed, which were proven effective in engineering applications.     

Effect of shape of nanoparticle on heat transfer and entropy generation of nanofluid-jet impingement cooling

ABSTRACT

Al₂O₃/water nanofluid has been numerically examined for the first time with different nanoparticle shapes including, cylindrical, blade, brick, platelet and spherical, on the flat and triangular-corrugated impinging surfaces. The volume fractions of 1.0%, 2.0% and 3.0% nanoparticles have been used. The Reynolds number is between 100–500 depending on the slot diameter. The finite volume method is utilized to determine the governing equations. The study is analyzed to determine how the flow features, heat transfer features and entropy production were affected by the diversity of nanoparticle shape, nanoparticle volume fraction, and shape of impinging surface. Darcy friction factor and Nusselt number are studied in detail for different conditions. The temperature contours are presented in the case of different nanoparticle volume fractions, nanoparticle shapes and both impinging surfaces. The results of the study suggest that the nanoparticle shape of the platelet shows the highest heat transfer development due to the thinner thermal boundary layer. Heat transfer augments with increasing volume fraction of nanoparticles. In addition, the study is consistent with the results of the literature on heat transfer and flow properties.     

Heat as emergency,heat as chronic stress: policy and institutional responses to vulnerability to extreme heat

Increasingly commonplace in cities, extreme heat events introduce multi-stress vulnerability, affecting people’s health and well-being, financial situation, mobility, social relations, and access to basic services. Planning to reduce heat vulnerability has become part of government business and to some extent community-level responses, cutting across a number of sectors including public health, emergency management, social services, critical infrastructure, and housing. This planning is often framed around heat as an emergency, focusing on preventing loss of life and severe health impacts, yet a vulnerability perspective also draws attention to the chronic and persistent impacts of heat. Our research, based on interviews and desktop research in Melbourne, Australia, found tensions between addressing heat as an emergency and heat as a source of chronic stress, with emergency responses taking precedence over responses addressing the chronic dimensions of heat. Each approach results in different but nonetheless related programmatic priorities for reducing vulnerability. In complex institutional settings, improving relations between policy and programme managers, non-government organisations, and vulnerable people themselves would enable the multiple stresses associated with extreme heat to be more effectively addressed. Policy and institutional responses that better appreciate the interconnections between the emergency and chronic aspects of heat would likely reduce vulnerability and contribute to more just approaches to urban sustainability.     

Modeling of Fluid Flow and Heat Transfer in a Copper Based Heat Sink Application

With the trend towards increasing the speed of processors in smaller sized of computers, there has been considerable interest in heat sink technologies with higher levels of performance and further miniaturization. This work addresses the fundamental heat transfer augmentation question of how to design a copper-based heat sink, when the overall dimensions of the bottom plate or fan are specified. A three-dimensional finite-volume model has been developed and applied to investigate flow and conjugate heat transfer in the copper-based heat sink. The model was produced with the commercial program FLUENT, which allows this nonlinear, highly turbulent problem to be simulated using the k-ε turbulence model. The theoretical model developed is validated by comparing the model predictions with available experimental data. The thermal performance and temperature distribution for the heat sink were analyzed and a procedure for optimizing the geometrical design parameters based on less space occupation and more efficient heat transfer coefficient is presented. Several design examples with different types of cooling methods and manufacturing processes have been analyzed. The reliability and effectiveness in heat spreading of those has been compared. It has been shown that the copper-based heat sink with louvered fins (case No.3) has an optimum design configuration.     

Investigation of modified horizontal wing cut twisted tapes fitted with rod and spacer at trailing edge on heat transfer properties in Double Glazing V-Trough solar water heaters

An experimental analysis of heat transfer efficiency in double glazing solar water heating system having full-length Horizontal wing cut twisted tapes having twist ratio 3 fitted with rod or spacer of dimensions 125 mm, 250 mm, and 500 mm as length at the trailing edges has been dealt in this work. The experimental results have been compared with a plain tube with similar operating conditions and concluded that variation in the Nusselt number, as well as friction factor, are ±12.56% and ±10.17%, respectively. The result of empirical correlation concludes that Nusselt number is maximal for horizontal wing cut twisted tapes having rod or spacer at its ends while comparing with full-length horizontal wing cut twisted tapes. The friction factor is low for full-length horizontal wing cut twisted tapes than the horizontal wing cut twisted tapes fitted with rod or spacer. The horizontal wing cut twisted tapes fitted with rod has superior performance than twist fitted with spacer.     

Heat transfer augmentation and flow characteristics in ribbed triangular duct solar air heater: An experimental analysis

An experimental study has been carried out to analyze the effect of inclined ribs used as roughness element on heat transfer along its friction characteristics. The roughness parameters include relative roughness pitch (P/e) ranges from 4–16, relative roughness height (e/D_h) ranges from 0.021–0.043 and angle of attack (α) ranges from 30° to 75°. The Reynolds number (Re) lies in the range of 4500–28000. The heat transfer and friction factor data obtained from the experiment and it is compared with the data obtained from smooth duct under the same conditions. Considerable augmentation was observed in heat transfer and friction over the smooth duct. Correlation was also developed for Nusselt number (Nu) and friction factor (f) as a function of roughness and flow parameter.     

油田注汽锅炉烟气余热利用与低碳减排

通过对注汽锅炉烟气余热利用潜力的分析及开展余热利用方式对比分析,采用在注汽锅炉对流段上安装热管换热器,使高温烟气与助燃空气换热,利用换热器回收烟气余热。该技术的现场应用,实现了注汽锅炉燃料单耗降低和污染物排放量降低的目的,取得了良好的节能、减排效果。     

Modeling of Flow Field and Heat Transfer in a Copper-Base Automotive Radiator Application

This paper describes a methodology used for designing louvered fins. Louvered fins are commonly used in many compact heat exchangers to increase the surface area and initiate new boundary layer growth. Detailed measurements can be accomplished with computational models of these louvered fins to gain a better understanding of the flow field and heat distribution. The particular louver geometry studies for this work have a louver angle of 23° and fin count of 17 fpi.

The flow and heat transfer characteristics for three-dimensional mixed convection flows in a radiator flat tube with louvered fins are analyzed numerically. A three-dimensional model is developed to investigate flow and conjugate heat transfer in the copper-based car radiator. The model was produced with the commercial program FLUENT. The theoretical model has been developed and validated by comparing the predictions of the model with available experimental data. The thermal performance and temperature distribution for the louvered fins were analyzed and a procedure for optimizing the geometrical design parameter is presented.

One fin specification among the various flat tube exchangers is recommended by first considering the heat transfer and pressure drop. The effects of variation of coolant flow conditions and external air conditions on the flow and the thermal characteristics for the selected radiator are investigated also. The results will be used as fundamental data for tube design by suggesting specifications for car radiator tubes.     

注汽锅炉烟气余热利用技术应用

通过对注汽锅炉烟气余热利用潜力分析和开展余热利用方式对比分析,采用了在对流段安装热管换热器,通过换热器将烟气余热与助燃空气换热,余热利用技术的现场应用,实现了注汽锅炉燃料单耗降低的目的,取得良好的节能效果。     

Performance investigation of a single-stage metal hydride heat transformer

This article presents the performance analysis of a single-stage metal hydride-based heat transformer (SS-MHHT) working with three different alloy pairs, namely LaNi_4.6Al_0.4/MmNi_4.15Fe_0.85, LaNi_4.61Mn_0.26Al_0.13/La_0.6Y_0.4Ni_4.8-Mn_0.2, and Zr_0.9Ti_0.1Cr_0.9Fe_1.1/Zr_0.9Ti_0.1Cr_0.6-Fe_1.4. The performances of the SS-MHHT are predicted by solving the conjugate heat and mass (hydrogen) transfer equations in cylindrical coordinates. The effects of various parameters such as heat output (T_H), heat input (T_M), and heat sink (T_L) temperatures on the coefficient of performance (COP_HT), specific heating power (SHP) and second law efficiency (η_E) are presented. The effects of overall heat transfer coefficient and mass ratio on the coefficient of performance (COP_HT) and specific heating power (SHP) are also presented. Numerical results are compared with the experimental data reported in the literature, and a good agreement is found between them. The maximum COP_HT of 0.436 and SHP of 54 W/kg are obtained for LaNi_4.61Mn_0.26Al_0.13/La_0.6Y_0.4Ni_4.8-Mn_0.2 pair. For a given operating temperatures of T_M = 358 K and T_L = 298 K, the maximum temperature lift of about 50 K is predicted for Zr_0.9Ti_0.1Cr_0.9Fe_1.1 /Zr_0.9Ti_0.1Cr_0.6Fe_1.4 pair.     

Canopy slope effect on the performance of the solar chimney power plant

Solar chimney power plants constitute an impressive construction by its size and its output seems low for its dimensions. Although many works have been carried out on the subject, there is still much scientific and technical improvement to be done. In the present work, we consider the modeling of turbulent flow under the effect of natural convection within a solar chimney power plant (SCPP) by performing numerical simulation using the Saturne Code coupled with Syrthes code. The objective of the study is the analysis of the collector cover slope influence on the performance of the SCPP in two cases. In the first case, the storage system considered is composed solely of the ground under the collector. For the second case, the storage system is made, in addition to the ground, of a 10 cm thick tub filled with water, covering the entire surface of the collector. The concept of minimizing the entropy production is also studied with the objective of optimizing the geometric configuration as well as the effect of the collector cover slope on the efficiency of SCPP. The boundary conditions are defined according to the meteorological data for a typical day available on the site of Adrar, Algeria. The results allow us to focus on the storage system influence on the SCPP performance and the duration of its operation after sunset. This leads to the improvement of the global efficiency of the SCPP. Results show the positive impact of the extra storage media use and the configuration which improves the velocity at the chimney entrance.     

Numerical studies on the thermal regimes of the horizontal tube falling film evaporation under varying feeder height

ABSTRACT

This paper discusses about the effect of feeder height and heat flux on the heat transfer characteristics of horizontal tube falling film evaporation in the thermal regimes. In order to investigate this, a two- dimensional CFD model was developed to perform simulation and results were compared and validated with published data available in the literature. Heat transfer co-efficients in the thermal regimes were determined from the CFD simulation and the results were recorded, analyzed and validated with the mathematical models available in the literature. The novelty of the current study is to predict the commencement of the fully developed thermal region over the tube from the simulation model under varying feeder height and heat flux. An effort was also made to measure the liquid film thickness around the tube from the CFD model in the thermal regimes. It is observed that angle of thermally developing region contracts and fully developed thermal region extends with the increase of the feeder height and heat flux. It is observed from the study that increase of heat flux by 10 kW/m² resulted in increase of heat transfer co-efficient value by 10–12% average in thermally developing region and 12–15% average in fully developed region. Thinnest liquid film thickness observed between 85 and 127°angle. Shifting of thinnest region of liquid film upward from the mid tube with the increase of the feeder height and heat flux is noted.     

The influences of recycle effect on double-pass V-corrugated solar air heaters

The study of the heat transfer enhancement for the recycling double-pass V-corrugated solar air heaters, which implement the external recycle of flowing air, was investigated experimentally and theoretically. The comparison among different designs of V-corrugated, baffled and fins attached, and flat-plate collectors was made to show the device performance improvement with various operating parameters under the same working dimensions. The recycling double-pass V-corrugated device developed here was proposed in aiming to strengthen the convective heat-transfer coefficient and enlarge the heat transfer area. The error analysis of experimental results deviate by 0.85–2.46% from the theoretical predictions with the fairly good agreement, and both results show that the device performance of the recycling double-pass V-corrugated operation is better than those of the other configurations under various recycle ratios and mass flow rates. The suitable selections were obtained for operating recycling double-pass V-corrugated devices while considering with an economic viewpoint by both the collector efficiency enhancement and the power consumption increment.