Heat Transfer Fluid Velocity Research Articles

The Effect of Heat Transfer Fluid Velocity on Heat Exchange Efficiency in Cold Energy Storage Tank: A Numerical Simulation Study

Developing a cold thermal energy storage (CTES) technology is one of the most effective methods to solve energy shortage and environmental pollution all over the world. The current study deals with the modelling and simulation of a cold thermal energy storage tank consisting of an polyvinyl chloride pipe (PVC) heat exchanger partially filled with a phase change material (PCM). Water, as the heat transfer fluid (HTF), flows through the inner tubes and the outer one while propylene glycol as the phase change material fills. This paper focuses on studying the effect of the velocity characteristics on the heat transfer efficiency of polyvinyl chloride pipe (PVC) heat exchanger in cold thermal energy storage system by the numerical simulation. In this paper, the detail of heat transfer performance within the heat exchanger is numerically solved using computational fluid dynamics (CFD), for various velocity as well as different heat transfer for optimal design. Several results of changes in the temperature field at the outlet of the cold thermal energy storage tank are presented when the inlet water velocity changes from 1 m/s to 1.4 m/s. The results indicate that low input water velocity will provide better heat exchange efficiency. However, it is required to make sure that the flow inside the heat exchanger is the turbulent flow because the study uses turbulent flow modules.

Study on the energy charging process of a plate-type latent heat thermal energy storage unit and optimization using Taguchi method

To understand the energy charging process of a plate-type thermal energy storage unit using phase change materials (PCMs) under various conditions, an experimental system was fabricated to identify the impact of heat transfer fluid (HTF) temperature, HTF velocity and plate inclination. The energy charging cycles were studied using liquid fraction and accumulated energy storage density with detailed temperature profiles. After the Taguchi optimization, a maximum energy charging rate of 759 W with a 23% increment was obtained with the HTF temperature of 55 °C, the HTF velocity of 5 m/s and the plate inclination of 75°. The melting process was influenced the most by the HTF temperature, followed by the HTF velocity and the plate inclination based on the standardized melting time. However, the influence of inclination angle was larger than the HTF velocity according to the actual melting time. Optimizing inclination angle was an effective way to improve the melting process without consuming additional materials and energy. Because of the PCM natural convection, the melting temperature range varied under different conditions. It was critical to identify the actual melting temperature range to predict the melting performance accurately.

Integrative passive and active cooling system using PCM and nanofluid for thermal regulation of concentrated photovoltaic solar cells

An integration of passive and active cooling systems for thermal regulation of concentrated photovoltaic (CPV) solar system has been developed and modeled. A heat storage battery of phase change material (PCM) is combined with a closed loop water cooling system in the developed design. A two dimensional model has been developed for the simulation of the CPV layers with the integrated cooling system. Results have been investigated to evaluate the thermal and electrical performances of the system components and the entire system. Further investigations have been conducted to study the effects of different arrangements of the PCM plates in the water tank on the system performances. Enhancing the system performance by using nanofluid as heat transfer fluid (HTF) was also evaluated. Results showed that the proposed system achieves 60% reduction in the CPV average temperature compared to the conventional direct PCM-PV and water-cooling individual systems. At 10 concentration ratio (CR) and 0.01 m/s HTF velocity, the cell temperature does not exceed 78 °C. Moreover, the PCM maximum temperature is kept below the degradation temperature limit. The effect of the PCM plates’ arrangements in the water tank on the system performance is negligible. Using nanofluid as HTF enhancer increases the CPV efficiency by 2.7% and reduces the PV maximum temperature and the PCM melting time by 4 °C and 12%, respectively.

Numerical simulation of heat release performance of filling body under condition of heat extracted by fluid flowing in buried tube

It is the basic requirement of the synergetic exploitation of deep mineral resources and geothermal resources to arrange the heat transfer tube in filling body. The heat release performance of filling body directly impacts on the exploiting efficiency of geothermal energy. Based on heat transfer theory, a three-dimensional unsteady heat transfer model of filling body is established by using FLUENT simulation software. Taking the horizontal U-shaped buried pipe as research object, the variation of temperature field in filling body around buried pipe is analyzed during the heat release process of filling body; the initial temperature of filling body, the diameter of buried pipe, the inlet temperature and inlet velocity of heat transfer fluid influencing of coupling heat transfer, which exists between heat transfer fluid and surrounding filling body within a certain axial distance of buried tube, and influencing of temperature difference between inlet and outlet of heat transfer fluid and on heat transfer performance of filling body are also discussed. It not only lays a theoretical foundation for the synergetic exploitation of mineral resources and geothermal energy in deep mines, but also provides a reference basis for the arrangement of buried pipes in filling body as well as the selection of working conditions for heat transfer fluid.

Investigation of charging and discharging characteristics of a horizontal conical shell and tube latent thermal energy storage device

In this study, a numerical model to analyse the charging and discharging characteristics of a horizontal shell and tube type Latent Heat Storage (LHS) prototype is presented. The system comprises of Sodium Nitrate as phase change material (PCM) in the shell side and flow of air as heat transfer fluid (HTF) in the tube side. The effective heat capacity approach is followed while solving the fluid flow and energy interactions in the PCM. The design is optimized by modifying a cylindrical shell into a conical shell heat exchanger system. The optimized values of diameters at inlet and outlet of the conical shell are found to be 98.6 mm and 54 mm, respectively having a cone angle of 3.4°. A performance comparison is made with a cylindrical shell system of equivalent storage capacity. 3-D numerical simulations performed on the proposed system revealed that an innovation in the design leads to enhanced heat transfer rate caused by uniform melting throughout the system. Further, the conical shell model is numerically simulated for various operating parameters such as inlet HTF velocity and temperature. Also, the effect of fins attached on the HTF tube on the performance of the system is analysed. With the advent of advanced heat exchanger design, there is a greater scope of obtaining higher heat transfer rates employing the proposed conical shell and tube LHS system.

Numerical investigation of heat transfer characteristics in a shell-and-tube latent heat thermal energy storage system

This paper presents the numerical investigation for a shell-and-tube latent heat thermal energy storage system (LHTES) employing numerical modeling and simulation with the FLUENT software. The LHTES consist of coilers where the heat transfer fluid (HTF) flows and a cuboids shell filled with phase change material. The unsteady N-S equation and energy equation using enthalpy measurement to deal with the latent heat of melting/solidification are solved. The effect of the tube diameter on the heat charging performance is examined, and the result shows that the functional relationship between the diameter of the tube and phase-change-rate is not monotonic. The optimal diameter of the tube at 20mm is used in all the subsequent computations. The influence of natural convection on charging and discharging processes is analyzed with positive tube pitch, and the result shows that natural convection plays a crucial role in the charging process and significantly accelerates the melting process; thermal conduction is dominant in the discharging process during the phase change stage which makes the discharging process much slower than the charging. The effects of the initial inlet temperature and velocity of HTF on the charging and discharging processes are studied in the case while PCM has a specific initial temperature. The result indicates that the inlet HTF velocity does not show a significant effect on the charging and discharging processes; the inlet HTF temperature shows a substantial effect on both processes. The temperature changing processes with time at different locations are also analyzed to dig more profound characteristics of the phase change process.

A MCRT-FVM-FEM coupled simulation for optical-thermal-structural analysis of parabolic trough solar collectors

Abstract In this work, a three dimensional optical-thermal-structural multi-field analysis model is established via a Monte Carlo Ray Tracing Method (MCRT)-Finite Volume Method (FVM)-Finite Element Method (FEM) coupled simulation method. Firstly, the localized concentration ratio (LCR) of a parabolic trough collector (PTC) is obtained by a MCRT method and the 3D non-uniform heat flux on the out surface of PTC absorber is calculated. Furthermore, the hydraulic and thermal analysis are realized through the FVM with previous obtained non-uniform heat flux conditions. Finally, the displacement and thermal stress in the PTC are calculated via FEM. The numerical results are compared with experimental data and good agreement is obtained, proving that the proposed numerical model is feasible. In addition, the effect of the inlet velocity of heat transfer fluid on the thermal-structural performance of PTC was discussed. The average Nusselt numbers, friction factor, deformation and thermal stress in the absorber was analysis.

Experimental investigation of a Cast-Steel based Thermal Energy Storage System

In the present work, the performance of a thermal energy storage module made of cast steel is evaluated using air as the heat transfer fluid (HTF). A cast steel module of length 740 mm and diameter 267 mm is fabricated with 19 cylindrical channels of diameter 12.7 mm for HTF flow. The module charging and discharging performance data are reported for an operating temperature range of 393.15 K to 473.15 K and the effect of HTF flow velocities on module performance is analysed. The estimated maximum storage capacity of the cast steel module is 13.76 MJ. The charging and discharging temperature profile across the module is measured to analyse the heat transfer phenomenon and it is observed that conduction varies predominantly in the axial direction, whereas, there is minimal variation in the radial direction. From the experimental results, it is comprehended that both charging and discharging rates are enhanced by increasing the HTF velocity from 2.5 m/s to 4.5 m/s. For a velocity of 4.5 m/s, the average module temperatures achieved at steady state condition for charging and discharging are 468 K and 398 K respectively.

Numerical Investigations on Charging/Discharging Performance of a Novel Truncated Cone Thermal Energy Storage Tank on a Concentrated Solar Power System

Developing a concentrated solar power (CSP) technology is one of the most effective methods to solve energy shortage and environmental pollution all over the world. Thermal energy storage (TES) system coupling with phase change materials (PCM) is one of the most significant methods to mitigate the intermittence of solar energy. In this paper, firstly, a 2D physical and mathematical model of a novel truncated cone shell-and-tube TES tank has been proposed based on enthalpy method. Secondly, the performance during the charging/discharging process of the truncated cone tank has been compared with the traditional cylindrical tank. Finally, the effects of inlet conditions of heat transfer fluid (HTF), and thickness of tube on the charging/discharging process, stored/released energy capacity; energy storage/release rate and heat storage efficiency have been investigated. The results show that the performance of truncated cone tank is better, and the charging/discharging time reduces 32.08% and 21.59%, respectively, compared with the cylindrical tank. The effect of wall thickness on the truncated cone TES tank can be ignored. And the inlet temperature and velocity of HTF have the significant influence on the charging/discharging performance of TES tank. And the maximum heat storage efficiency of the truncated cone TES tank can reach 93%. However, some appropriate methods should be taken for improving the thermal energy utilization rate of HTF in the future. This research will provide insights and significant reference towards geometric design and operating conditions in TES system.

Thermal Energy Storage Using Horizontal Shell-Tube Heat Exchanger: Numerical Investigation on Temperature Variation of HTF

Abstract- Latent Heat Thermal Energy Storage (LHTES) is a method to store thermal energy in a Phase Change Material (PCM). Due to the higher energy density, the efficiency of the size of the container might happen. However, the thermal energy storing (charging) time on LHTES is considerably inefficient. The increasing velocity of HTF was conducted using a modified fin and encapsulation to improve the charging time. Similarly, with this motivation, numerical analysis with modified density modelling was conducted to investigate the effect of temperature of heat transfer fluid (HTF) on PCM. This investigation was validated with shell-tube geometry experiment. The experiment set-up was divided into two parts where shell part used as circulation of Heat Transfer Fluid (HTF) and tube part containing PCM (RT-52). This simulation carried out using ANSYS FLUENT 17 to observe evolution on radial-axial temperature, melting time, melting contour and natural convection as the effect of HTF temperature variation. The natural convection got higher and induced the circulation in tube as well. Thus, the melting area was much more at the top cylinder. Melting time decreased significantly as the rising of HTF temperature.

EXPERIMENTAL ANALYSIS AND TRANSIENT SIMULATION OF HEAT TRANSFER INSIDE THE FINNED TUBE ADSORBENT BED OF A THERMAL WAVE CYCLE

Heat transfer enhancement inside the adsorbent bed of a thermal wave adsorption cooling cycle is investigated both experimentally and theoretically. Various adsorbent materials are tested using a finned tube adsorbent bed for the thermal wave cycle. The mathematical model is well defined, including a 2-D coupled heat and mass transfer analysis. This study presents the effects of heat transfer fluid velocity, regeneration temperature, condenser pressure, and particle diameter on the heat transfer enhancement inside the bed. For verification of the theoretical model, temperature measurements were taken in both radial and axial directions. As a result of the experimental study, a good agreement is achieved between the predictions and experiments.

Influence of PCM thermal conductivity and HTF velocity during solidification of PCM through the free cooling concept – A parametric study

Abstract The thermal performance of a latent heat thermal energy storage (LHTES) system can be enhanced by incorporation of fins, metal matrices, lessing rings in the phase change material (PCM) encapsulation and by dispersion of high conductive nanomaterials in the PCM itself. Similarly, to increase the heat transfer to PCM, the surface convective heat transfer coefficient (‘h’) can be enhanced by increasing the heat transfer fluid (HTF) velocity (‘u’). However, it is important to know how much increase in PCM thermal conductivity (‘k’) and ‘h’ will be beneficial during the charging of PCM under real-time ambient conditions and when will we reach the margin of diminishing returns. Understanding these are the motivation of the present work which includes the parametric study of the impact of ‘k’ & ‘u’ on the charging of PCM (RT28HC) through free cooling concept under different operating conditions (two different HTF inlet temperature). The numerical results are validated using the experimental data and they both show good agreement with each other. The major inferences from the results are, i) increasing the PCM thermal conductivity reduces the charging duration of the LHTES system for both lower and higher HTF velocity (1 m/s and 8 m/s). However, reduction in charging duration while increasing ‘k’ is higher when the HTF inlet temperature is lower, ii) increasing the HTF velocity is beneficial only when the inlet HTF temperature is higher. For the case of lower HTF inlet temperature, the effect of increasing the HTF velocity is suppressed by the higher temperature driving potential between the HTF inlet temperature and the PCM phase change temperature. It is inferred from the parametric study that, among the three parameters considered (PCM thermal conductivity, HTF velocity, and HTF inlet temperature), HTF inlet temperature has the most influence on the charging of the PCM, followed by the PCM thermal conductivity and HTF velocity.

Simulated performance analysis of a solar aided power generation plant in fuel saving operation mode

Solar aided (coal-fired) power generation (SAPG) system has been proved to be an efficient way to utilize the solar energy for power generation. Due to the instability of the solar radiation, a SAPG system generally operates under transient working conditions. In this paper, performance simulation sub-models of main components in a SAPG plant are established based on the lumped parameter assumption. A 330 MW SAPG power plant as a case study is simulated. The variations of the performances, main parameters of the plant with the solar field heat output and the dynamic responses under a typical day are analyzed. The results show that when the heat output of the solar field changes from 0 kJ/h to 2.13 × 108 kJ/h, the coal saving rate will increase to 6.4%, and the solar power generation share (the proportion of the power from the solar energy to the total power from the SAPG plant) will increase to 7.74%. During the analysis process, in order to optimize the solar field, the concept of the solar field equivalent efficiency (SFEE) is proposed and the optimal velocity of heat transfer fluid (HTF) in absorber tube is obtained.

Enhancement in free cooling potential through PCM based storage system integrated with direct evaporative cooling (DEC) unit

The present work reports the enhancement in free cooling potential using a modified cooling system compared to the conventional free cooling system. The proposed modified system is a novel pilot scale model packed with spherically encapsulated phase change material in a cylindrical tank along with water spray nozzles (direct evaporative cooling unit) at the inlet of the tank. The experiments were conducted in Bangalore, a city located in south India that possesses moderate/temperate climate throughout the year. Considering the local ambient conditions, an organic phase change material with the phase transition temperature range of 25.6–27.1 °C was used in the present study. Significant reduction in total charging duration and enhancement in heat transfer rate was achieved through the hybrid cooling system. The reduction in charging duration of 28.7% and 34.8% was observed for the proposed hybrid cooling system at heat transfer fluid (HTF) inlet velocities of 2 and 1.5 m/s respectively. It is observed from the results that in the experiments conducted with conventional free cooling system at 1 m/s HTF velocity, the phase change material (PCM) placed in the last two rows of the storage tank did not reach its end freezing temperature even after 10 h of experimentation due to the low heat transfer rate, whereas in the experiments conducted with the modified free cooling system, the storage tank is completely charged at all HTF velocities. It is construed from the results that the integration of evaporative cooling unit along with phase change material based free cooling system aids the chosen phase change material to completely solidify at a faster rate and augments the thermal performance of the storage unit. The proposed system can be operated as a single stand-alone cooling system to meet the cooling demand of the buildings or it can be integrated with the mechanically operated HVAC systems to achieve energy efficiency in the overall cooling system.

Heat transfer analysis of a new volumetric based receiver for parabolic trough solar collector

Abstract Effective use of selective absorbing coating in surface based receivers and nanofluids in volumetric based receivers for better thermal performance have been confirmed in previous studies. However, in practical application, these receivers suffered from the problem of performance degradation after long-term high-temperature operation, and they were high-cost. This study proposed a novel kind of parabolic trough receiver by locating the absorber tube inside heat transfer fluid (HTF) in a twin glass tube, and developed its mathematical heat transfer model. Based on this model, the novel receiver was verified to be desirable when the inlet temperature of HTF was lower than 150 °C (with an efficiency sacrifice within 4%), and particularly in the range of 100–120 °C (with an efficiency sacrifice within 1.5%). Strategies including replacing the selective absorbing coating with non-selective absorbing coating, lowering the velocity of outer HTF and lowering the emissivity of the inner glass tube were investigated and verified to be effective in specific conditions.

Modeling and investigation of high temperature phase change materials (PCM) in different storage tank configurations

In recent years, one of the most actual issues is improvement of the energy-saving technologies. Using specific materials in the specific temperature range as phase change material (PCM) allows using latent heat for storage thermal energy. This is one of the most efficient ways of storing thermal energy. In this paper, charging process of high temperature PCMs in three different configurations is modeled. Ten different PCMs which are Sodium, Potassium and Lithium salts are compared and three configurations of latent heat thermal energy system units for each PCM are investigated. Effect of the heat transfer fluid (HTF) velocity is also examined. The time required for charging the tank for the considered PCMs is calculated. Among these materials LiF(33)-67KF requires just 15 h for completely charging while NaCl(33)-67CaCl2 needs more than 30 h. Results show that ratio of surface area to volume of PCM domain plays an important role in tank charging time and increasing HTF velocity decrease charging time but not always.

Numerical analysis of a near-room-temperature magnetic cooling system

In this study, for a near-room-temperature magnetic cooling system, a decoupled multi-physics numerical approach (Magnetism, Fluid Flow, and Heat Transfer) is developed using a commercial CFD solver, ANSYS-FLUENT, as a design tool. User defined functions are incorporated into the software in order to take into account the magnetocaloric effect. Magnetic flux density is assumed to be linear during the magnetization and demagnetization processes. Furthermore, the minimum and maximum magnetic flux densities (Bmin and Bmax) are defined as 0.27 and 0.98, respectively. Two different sets of analyses are conducted by assuming an insulated cold heat exchanger (CHEX) and by defining an artificial cooling load in the CHEX. As a validation case, experimental work from the literature is reproduced numerically, and the results show that the current methodology is fairly accurate. Moreover, parametric analyses are conducted to investigate the effect of the velocity of heat transfer fluid (HTF) and types of HTF on the performance of the magnetic cooling system. Also, the performance metrics of the magnetic cooling system are investigated with regards to the temperature span of the magnetic cooling unit, and the cooling load. It is concluded that reducing the cycle duration ensures reaching lower temperature values. Similarly, reducing the velocity of the HTF allows reducing the outlet temperature of the HTF. In the current system, the highest temperature spans are obtained numerically as around 6 K, 5.2 K and 4.1 K for the cycle durations of 4.2 s, 6.2 s and 8.2 s, respectively.

A new solution for thermal interference of vertical U-tube ground heat exchanger for cold area in China

This paper presents a new solution to reduce the thermal interference of vertical U-tube ground heat exchanger (UGHE): pipe insulation installed on the upward branch pipe of the U-tube. 3-dimensional numerical models of the UGHE with and without pipe insulation were developed to simulate the heat transfer process around the UGHE. The models were verified with previously published data. The effects of pipe insulation on outlet temperature, soil temperature, heat exchange rate per unit length of borehole depth and soil temperature recovery ratio were investigated. The effects of inlet velocity of heat transfer fluid (HTF) and the thermal conductivity of backfill material were also studied to fully assess the effects of the insulation. The results indicate that the average outlet temperature of HTF, soil temperature near the upward branch pipe and soil temperature recovery ratio all increased and the thermal interference can be effectively weakened when pipe insulation was installed.

Heat transfer performance of thermal energy storage components containing composite phase change materials

This study concerns about the heat transfer behaviour of composite phase change materials (CPCMs) based thermal energy storage components. Two types of components, a single tube and a concentric tube component, are designed and investigated. The CPCMs consist of a molten salt based phase change material, a thermal conductivity enhancement material (TCEM) and a ceramic skeleton material. A mathematical model was established to model the heat transfer behaviour. The modelling results were first compared with experiments and reasonably good agreement with the experimental data was obtained, demonstrating the reliability of the model. Extensive modelling studies were then carried out under different conditions. The influence of thermoproperties, surface roughness and size of the CPCMs as well as heat transfer fluid (HTF) velocity were examined. The results show that the thermal contact resistance between the CPCMs should be considered. Increasing the mass fraction of TCEMs and thickness of CPCMs as well as the HTF velocity intensifies the heat transfer behaviour of component. The concentric tube based component offers a better heat transfer performance compared with the single tube based component, with the total heat storage and release time ∼10 and 15% shorter, respectively, for a given set of conditions.

Application of an embossed plate heat exchanger to adsorption chiller

The present research conducted a parametric study on an embossed plate heat exchanger (plate HX) type adsorption chiller with SWS-1L and water pair, using a numerical method. The plate HX has a relatively high heat transfer capacity and compact size, and this study is a first attempt to apply the plate HX as a new type of adsorption chiller, as an improved alterative to the fin-tube type heat exchanger. A feasibility study was conducted on the base model and the result is comparable to the value for existing fin-tube type adsorption chillers. Furthermore, a parameter study was conducted for seven important design parameters, embossing diameter ratio, embossing height, embossing pitch, bed thickness, plate thickness, heating temperature and heat transfer fluid velocity. The results provided guidelines for the optimal designs of plate HX type adsorption chiller. The optimal values of COP and SCP were 0.5118 and 662.8 W kg−1, respectively.