Fluid Inlet Temperature Research Articles

Design and investigation of single tank phase change thermal storage domestic hot water system

Thermal energy storage (TES) is extensively applied in production and daily life. As a basic work, we designed a single tank phase change TES domestic hot water system using night valley power. Based on the single tank TES experimental platform, a 2D numerical model of the system was established. To optimize the performance of a single tank TES system, the effect of heat transfer fluid (HTF) inlet velocity and temperature on the heat release of the storage tank was analyzed based on simulation. The enhanced heat transfer effect of the radial cascade system and axial cascade system was also compared. The results showed that 0.8 m/s is the optimal HTF inlet velocity in the calculation range. The rule of the heat release about the HTF inlet temperature just meets the different hot water requirements in winter and summer. The heat release capacity of the radial cascade system and axial cascade system is higher than that of the conventional single-stage system, and the proportion of latent heat release in radial cascade is larger. This study will provide theoretical guidance for the optimal design of the single tank TES domestic hot water system in the future.

Non-Linear Qualitative Dynamic Analysis of Supercritical Water-Heated Channels under External Vertical Accelerations

Employing the external force method to regard seismic impact and the three-region methodology to analyze the supercritical heated channel, a non-linear dynamic model was developed to investigate the transient characteristics of single channel or parallel channels under the impacts of vertical sinusoidal and seismic accelerations. The present model was validated against the experimental data, which could suitably estimate the additional pressure drop caused by the vertical vibrations. The influences of parameters on the seismic-induced oscillation conducted in a supercritical heated channel indicated that a longer heated length, uprating operation power and a larger outlet loss coefficient all exhibit unstable effects, while the increase of inlet loss coefficient, a larger tube diameter and a lower inlet fluid temperature would tend to stabilize the system. Moreover, the supercritical fluid would present a high natural frequency in the very small NP-SUB region. The parametric effects on the parallel channel system are related to the inherent stability nature of initial state and the interactions among channels. The more uneven heat flux distribution among channels would cause a larger vibration-induced oscillation. In particular, when it is combined with the resonance effect, the system may exhibit much larger oscillations than in the case of non-resonance.

Numerical study on the thermo-hydraulic performance analysis of fly ash nanofluid

The heat transfer and flow characteristics of water-based fly ash nanofluid for concentrations of 0.1–1.5 vol.% that flow through a copper tube at an inlet fluid temperature of 303 K with constant heat flux boundary conditions are estimated numerically. Stability, viscosity, and thermal conductivity of fly ash nanofluid have been determined experimentally and compared with correlations available in the literature. STAR CCM + software was used to solve the governing equations using the finite volume method (FVM). Water and stable fly ash nanofluid were used as working fluids for the Reynolds number range of 4500–16,000. The Nusselt number and friction factor at 1.5% volume concentration are greater than the base fluid by 36% and 9.4%, respectively. The fly ash nanofluid showed a performance index ( $$\eta$$ ) value of more than 1 at all concentrations undertaken, indicating that the use of fly ash nanofluid in heat transfer applications is beneficial. The highest $$\eta$$ value is determined to be 1.5 approximately for a nanofluid concentration of 1.5 vol.%. The fly ash nanofluid exhibits enhanced heat transfer performance contrasted to SiO2 nanofluid. The utilization of fly ash particles for heat transfer enhancement can aid in the reduction of environmental pollution to a certain extent. Novel correlations are suggested for the estimation of fly ash nanofluid Nusselt number and friction factor.

Experimental characterization of a prototype of bidirectional substation for district heating with thermal prosumers

The prosumer, already widespread in the electrical sector, is still uncommon in the district heating (DH) sector. Nevertheless, this figure can potentially provide a relevant contribution to increase the renewable fraction of heat and to decrease the fossil fuel consumption, hence enhancing sustainable and efficient district heating. Moreover, prosumers are more informed and responsible towards energy production and energy savings. In order to enable the two-way heat exchange, the thermal substation at the interface between the prosumer and the DH network must be properly upgraded.The present paper aims at providing a comprehensive contribution to the design and testing of an innovative bidirectional substation for active district heating, focusing on the hydraulic configuration and on the control strategies. The realized substation primarily fulfils the prosumer’s heat demand and supplies the excess heat to the DH network only if it is available at the temperature contractually defined with the DH operator, while it uses the network as a source if the local production is not sufficient to cover the user’s heat demand. The prototype, with a technology readiness level TRL 4, can be connected to a generic micro-generation system, e.g. solar thermal or heat recovery units. An extensive test campaign has been carried out in order to evaluate its dynamic behavior both from energetic and hydraulic points of view. Test confirmed the reliable operation of the substation, which is able to handle simultaneous heat exchanges with the user’s heating system and the DH network at pressure loss lower than 0.2 bar and at temperatures within ±0.6 °C of the prescribed setpoint after transients shorter than 5 min, even with ±10 °C step changes in the inlet fluid temperatures.

Analysis and optimization of triple tube phase change material based energy storage system

We present an analysis and optimization of triple tube phase change material (PCM) based energy storage system. The study considers a triple tube energy storage system with PCM stored in the inner cylinder and outer annular region while heated water is passed through the inner annular region. Simulations are performed to analyze the effect of inner cylinder diameter on the total time required for complete melting of PCM while keeping the volume of PCM constant. Results show that the triple cylinder configuration leads to considerably faster melting as compared to the double cylinder system for the same mass flow rate of water and same quantity of PCM. It is found that the inner diameter, inlet temperature and mass flow rate of heat transfer fluid have considerable effect on the melting time while the effect of cylinder length is negligible. The main outcome of the study is that it shows that there exists an optimum diameter of the inner cylinder for which the melting rate is highest and considerably faster than that for the maximum and minimum diameter. Subsequent studies for different mass flow rates of water show that the optimum diameter does not vary significantly with change in mass flow rate.

Study the flow direction and number of tubes in cross-flow heat exchanger to improve the heat transfer coefficient.

This study deals with the effects the number of tubes and flow direction on the heat transfer coefficient. This investigation complete by study different lengths of smooth copper tubes (50,100, 150, 200 and 250) mm with different passes (2, 4, 6, 8 and 10) for cross flow heat exchanger to attain the best length to heat transfer numerically, by Solid work 2017 and ANSYS 16.1 program. The diameters of outer and inner tubes are (24 and 19 mm), air velocity (3 m/s), volumetric flow rate (5 L/min) and the inlet temperature of cold and hot fluid (25 °C and 80°C) respectively. Also been studying the effect of a smooth and triangle finned tube (4 passes) to enhance the heat transfer. Moreover, studying the effect of up and horizontal direction of the flow on heat transfer, by designing smooth tube 8 passes heat exchanger with the same dimension, air velocity inlet (1 m/s), volumetric flow rate (6 L/min), the temperature of the cold fluid inlet (22 °C) and the temperature of the hot fluid (50 °C). It can be noted that whenever the number of passes of tubes increases, the amount of heat transfer increase at the expense of the increase in pressure drop. The finned tube gives better heat improvement than a smooth tube and the direction of flow does not effect on the heat transfer.

A technical, financial and CO2 emission analysis of the implementation of metal foam in a thermal battery for cold chain transport

Latent thermal energy storage (LTES) can be a valuable component in a wide range of energy systems. The low thermal conductivity of most phase change materials (PCMs) has resulted in the development of several PCM enhancement methods. However, most of the comparisons of enhancement techniques implemented in LTES heat exchangers are based on a single operating point which limits the findings to the tested heat transfer fluid inlet temperature and mass flow rate. Furthermore, an analysis of the financial impact of an enhancement method is not available. In this article, the validity of implementing metal foam as a thermal conductivity enhancer is assessed in a cold chain application. A modular LTES design is constructed with and without metal foam. The charging time of both thermal batteries is experimentally correlated as a function of the heat transfer fluid inlet temperature and mass flow rate. The metal foam reduces the charging time of the battery between 24 % and 40 % depending on the heat transfer fluid inlet condition. Discounted savings for the implementation of metal foam of up to 10 times the investment cost are possible but depend on the number of cycles per year and the rate of return.

Critical Heat Flux of Confined Round Single Jet and Jet Array Impingement Boiling

This study involves experimental investigation of key parameters influencing CHF for confined round single jets and jet arrays impinging normally onto square heated surfaces. The experiments are performed using R-134a, a fluid widely used for thermal management of electronic and power devices, especially in aerospace applications. A comprehensive R-134a CHF database is acquired that considers the effects of various geometrical parameters and operating conditions. Close examination of the data trends reveals several strategies to augment CHF, such as increasing jet velocity and/or total mass flow rate and employing larger jet diameters for a fixed velocity or smaller diameters for a fixed flow rate. Higher CHF is also achieved by increasing saturation pressure for a fixed inlet fluid temperature (i.e., higher saturation pressures combined with higher inlet subcooling). Fluid exit qualities point to two different CHF mechanisms: subcooled CHF at high flow rates and saturated CHF at low flow rates. Underlying mechanisms are also propounded for two types of CHF transients: a sudden sharp temperature escalation at lower flow rates and a mild gradual increase at higher flow rates. Close inspection of the heating surface following CHF tests shows localized burnout patterns which provide significant insight into both the flow characteristics within the confinement region and the spatial distribution of surface temperature resulting from jet interactions. Statistical inference techniques are used in conjunction with the new understanding of fluid flow and heat transfer physics to formulate a new correlation form for CHF. The resulting correlation, which is based on a consolidated database of the present R-134a and previous FC-72 data, shows good prediction accuracy, evidenced by a mean absolute error of 16.66% for both fluids and over broad ranges of geometrical parameters and operating conditions.

Experimental study on cooling performance of nanofluid flow in a horizontal circular tube

In this experimental study, the cooling performance of alumina/water nanofluid flow in a heated copper tube with constant heat flux is explored. The experiments are conducted for the nanoparticle's concentration in the range of 0 to 10%, the Reynolds number in the range of 168 to 2031, two values of inlet temperature of fluid, 25 °C and 50 °C, and the imposed heat flux in the range of 3979 to 7957 W.m − 2. The influences of these parameters on the friction factor, Nusselt number, temperature on internal wall of tube, the bulk temperature of fluid, the profit index, and the heat transfer effectiveness ratio are investigated. The experimental data and compared with the numerical results. The experimental results clearly exhibit a good agreement with the theoretical data existing in the literature and the numerical results. The outcomes indicate that for a constant value of nanoparticles concentration, the heat transfer effectiveness ratio and the profit index increase as the inlet temperature of fluid is increased. In addition, boosting the concentration of nanoparticles causes the increase in heat transfer effectiveness ratio. The maximum heat transfer effectiveness ratio of 1.105 is achieved by using the nanofluid. The maximum profit index of 1.065 can be achieved at the concentration of 10% and the inlet temperature of 50 °C. The heat flux has no considerable effect on the temperature on the internal wall of tube and the bulk temperature of fluid.

A spectral self-regulating parabolic trough solar receiver integrated with vanadium dioxide-based thermochromic coating

A significant decrease in the solar-thermal conversion efficiency of parabolic trough collectors occurs at high operating temperatures, mainly due to the massive radiant heat loss from the parabolic trough solar receiver incurred in this case. In this paper, a novel parabolic trough solar receiver integrated with vanadium dioxide-based thermochromic coating is proposed to effectively reduce the radiant heat loss and improve the overall performance of solar receivers. The thermochromic layer, deposited on the glass envelope’s inner surface in the negative thermal-flux region, has a reversible transition from a monoclinic (M) phase to a rutile (R) phase at the critical temperature of 68 °C. The occurrence of the phase transition in the thermochromic coating induces beneficial self-regulation of the spectrum-selectivity characteristics of the glass envelope, i.e., transmittance, reflectance, and absorptance (emittance) of glass envelope to solar and infrared radiation, for enhancing the thermal performance of solar receivers. Comprehensive heat transfer models based on the finite volume and spectral radiant heat transfer methods are established in this study. It is validated the models can predict the thermal performance of the solar receivers with high accuracy. In this framework, the total heat loss, thermal efficiency, and exergy efficiency of parabolic trough collector systems integrated with the proposed novel solar receivers are investigated and analyzed. Besides, the effects of transmittance of thermochromic coating (M phase) and emittance of thermochromic coating (R phase), two key parameters, on the overall performance of proposed solar receivers are also studied. The results show that no matter under the M phase or R phase, the thermochromic coating can play unique advantages to improve the thermal performance of the proposed solar receivers. The heat loss and thermal efficiency of the proposed solar receivers are effectively reduced and enhanced by 18.2% and 4.8% at the absorber temperature of 600 °C and inlet fluid temperature of 580 °C, respectively.

Study on Outlet Temperature Control of External Receiver for Solar Power Tower

Due to the change of direct normal irradiance (DNI) and the change of output power load, the receiver of the solar tower is in an unstable state in the actual operation. In this paper, a 100 MW external cylindric receiver is designed and modelled. The dynamic and comprehensive model is established for the receiver, including the thermal and mechanical equations. The temperature control strategy is applied to the receiver model. The validity of the control strategy is verified by disturbance experiments, including DNI, the inlet temperature of the heat transfer fluid (HTF), and the weather data on a cloudy day. The response characteristics of the receiver are demonstrated. Its thermal lag characteristics and restraining effect on the fluctuating environment are revealed. The dangerous occasion of the receiver during operation are detected, including the overheat of the local panel, and the dissociation point of the molten salt. Both the robustness and the deficiency of the control strategy of the receiver are pointed out. The research results will contribute to the control strategy formulation of the SPT (solar power tower) station.

Flat-Plate Solar Collector Thermal Performance and Optimal Operation Mode by Exergy Analysis and Numerical Simulation

In this paper, the effect of a flat-plate solar collector components exergy destruction rates on the collector performance has been examined. A theoretical model based on energy and exergy balance for glass cover, absorber plate and working fluid resulted in nonlinear ordinary differentials non-autonomous system of equations that was solved numerically. Upon verification of the accuracy of the proposed model with experimental data, the effect of parameters such as solar radiation, mass flow rate, inlet fluid temperature and insulation thickness on the exergy destruction rates and exergy efficiency has been investigated. The model was used to optimize parameters, such as inlet fluid temperature, mass flow rate and number of collector tube. The results reveal that the highest exergy destruction rate occurs in the absorber plate, which is 79.23% of the total exergy destruction rate. Increasing the mass flow rate to 0.0087 kg/s leads to a decrease in the absorber plate exergy destruction rate to a minimum value of 575.74 W/m2 and to an increase in the exergy efficiency to a maximum value of 21.97%. When the inlet fluid temperature increases from 20 to 50 °C, the absorber plate exergy destruction rate reduces from 676.66 to 438.40 W/m2 resulting in a significant increase in the collector exergy efficiency from 6.80 to 37.86%. The optimum operating condition was found to be 37 °C for the inlet fluid temperature, 0.0087 kg/s for mass flow rate and fifteen for the number of tubes.

Influence of Process Parameters on the Mechanical Properties of the Reformer Tube

Considering the non-linear characteristics of the reformer tube material with temperature changes and the influence of the fluid inside and outside the tube, we suggest to establish a thermal-fluid-solid coupling model and calculation method for the reformer tube of the hydrogen production reformer. The multi field coupling theory is used to analyze and study the stress distribution of the fluid in the reformer tube under different process conditions, different inlet temperature and different inlet flow rate.We found that under hot operating conditions, as the fluid inlet temperature in the tube increasing, the maximum equivalent stress of the overall reformer tube structure gradually decreased. As the flow rate gradually increasing, the maximum equivalent stress of the reformer tube decreased gradually and then increased. In this paper, all the technological parameters come from the actual production conditions, so the research results provide a certain reference for safe production.

Applying the Concepts of Efficiency and Effectiveness to Analyze the Influence of the Number of Passes in the Shell and Tubes Condenser Thermal Performance

The work analyzes the influence of the number of passes in a shell and tubes condenser heat exchanger, with an inlet pressure of R134a refrigerant in the shell equal to 1.2 MPa. The fluid that circulates in the tubes is water or water-based nanofluid with a fraction of aluminum oxide nanoparticles (Al2O3), and the methodology used subdivides the heat exchanger into three distinct regions: the overheated region, the saturated region, and the subcooled region. The main parameters used to analyze the thermal performance of the heat exchanger were efficiency and effectiveness. Efficiency in the superheated steam region is close to 1.0. There is scope for increasing thermal effectiveness, which can be improved with more significant passes in the tube. The saturated steam region process is efficient for lower mass flow rates of the fluid in the tube, but it is ineffective. However, it is highly effective for high mass flow rates. There is ample scope for increasing effectiveness in the subcooled region. Still, the fluid inlet temperature in the pipe and the work refrigerant pressure are the limiting factors for greater heat exchange in the subcooled region.

Experimental investigation and heat loss analysis of a three-coil solar cavity receiver of parabolic dish collector under wind condition

A modified three-coil solar cavity receiver is experimentally investigated to observe the effect of increases in cavity inner wall area, fluid inlet temperature (50°C-80°C) and cavity inclination (θ = 0 to 90°) on the total heat loss from receiver under no wind and wind condition. The hot water is supplied from the top of the receiver and total heat loss is estimated experimentally. The total heat loss is reduced up to 1.2723-1.91 times (at θ = 0°-90°) as compared to single coil receiver at wind velocity 3 m/s. At wind velocity 3 m/s-5 m/s, the total heat loss increases up to 1.238-3.41 times at θ = 0° and 1.53-1.68 times at θ = 90°. The developed analytical model, estimates 1.52%-3.23% conduction, 1.36%-2.64% radiation and 97.11%-94% convection heat loss. With increase in cavity inclination the convection heat loss decreases sharply, while the radiation and conduction heat losses increase slowly.

Experimental study on the charging and discharging behaviour of capric–lauric acid/oleic acid mixture in a cold thermal energy storage system for cold storage applications

In this study a capric–lauric acid/oleic acid mixture was synthesized to form a low temperature PCM for cold storage applications. An experimental investigation was carried out in a double helical coil cold thermal energy storage unit (CTESU) to study the charging and discharging behaviour of capric–lauric acid/oleic acid mixture. The inlet temperature and flow rate of the heat transfer fluid (HTF) were selected as experimental parameters to analyse the cold thermal energy storage performance of PCM. The analysis includes charging and discharging behaviour, energy dissipation rate, energy storage rate, energy charging efficiency, energy discharging efficiency and influence of HTF inlet temperature and flow rate under varied experimental parameters. With the increased contact area, the developed double helical coil considerably improves the heat transfer performance of the cold thermal energy storage unit. The discharging rate was 18% faster than the charging process due to predominance of natural convection. The PCM solidification during charging process is controlled by conduction. With increasing HTF flow rate during the discharging process, the melting rate was 16% higher than lower flow rates. In contrast, as flow rates increased, the charging rate did not change significantly. The role of natural convection and stratification which leads to unnecessary mixing of PCM consequently lowering the cooling energy stored. Also results suggest that capric–lauric acid/oleic acid can be utilized as a potential PCM for low temperature storage applications.

Effect of Inlet and Geometrical Parameters on the Melting of PCM Capsules of Elliptical Cross Section.

This paper focuses on the melting of phase change material capsulated in the elliptical cross-section horizontal cylinder under convective boundary conditions. Different parameters are discussed, namely, the HTF inlet temperature and velocity and the axes ratio of the capsule cross-section. The effect of HTF inlet temperature, inlet velocity, and the capsule axes ratio was studied using the CFD software FLUENT 6.3.26. A comparison between the numerical and experimental results was made for the system. The experimental results matched well with the heat transfer model. It is shown that the inlet temperature of the heat transfer fluid (HTF) has a great effect on the process of paraffin wax melting. The increase of HTF inlet temperature increases the Phase change material( PCM) liquid fraction at constant charging time and decreases at the same time the total time of the capsulated paraffin wax melting. The geometry of the capsule cross-section represented by its axes ratio has a sensible effect on the process of paraffin wax melting. Increasing the axes ratio of the capsule, i.e. elongated the capsule in the perpendicular direction of flow increasing the PCM liquid fraction at constant charging time and decreases the total time of the capsulated paraffin wax melting. The increase of HTF inlet velocity has a weak effect on the process of paraffin wax melting in the inlet velocity range 0.003-0.012 m/s.

Performance optimization of latent heat storage by structural parameters and operating conditions using Al-based alloy as phase change material

Heat storage technology can effectively solve the intermittency and instability of solar radiation and it also plays a vital role in solar thermal power generation. In this paper, Al-based alloys as candidates for high-temperature phase change material (PCM) with different Si/Cu content ratios are prepared. Thermal properties such as melting point, latent heat, specific heat, and thermal conductivity are investigated. A numerical model of phase change heat storage unit (PCHSU) is developed to analyze the heat transfer characteristics. The structural parameters are optimized by the PCM container structure, the surface to volume ratio, and inserted inner ring fins. The operating conditions are optimized by the flow direction, the inlet temperature, and the velocity of the thermal fluid. The results show that Al-5%Si-31.5%Cu (wt. %) has strengths in latent heat, specific heat, and thermal conductivity. The PCM container with a circular structure is superior to that of hexagonal and square structures in heat storage rate and thermal uniformity. The heat storage rate of PCHSU with inner ring finned tube is 6.83% higher than that of the smooth tube, and it has the advantage of inhibiting thermal stress due to the axial temperature uniformity. The horizontal PCHSU is more helpful in stabilizing the state of thermal fluid outlet temperature. Notably, 1023 K and 10 m/s are the optimal inlet temperature and velocity of the thermal fluid. Finally, the heat storage module is analyzed based on the variation of solar radiation intensity. These conclusions provide guidance for the design and optimization of PCHSU for heat storage systems.

Experimental Investigation on the Effect a Rotational Shaft on the Thermal Behavior of a Circular Tube under Constant Heat Flux

Active and passive methods are two main mechanisms of heat transfer improvement. The active methods use external forces to improve heat transfer. This investigation evaluates the thermal and frictional behavior of a circular tube containing a rotational shaft. Constant heat flux was exerted to the circular tube. The fluid inlet and outlet temperature as well as wall temperature of tubes were measured to calculate the hat transfer coefficient. The Re (Reynolds) number was between 800-2000. Also, the dimensionless rotational speed (Rs) had the values of 1,1.5, 2, 2.5 and 3. Results revealed that the rotational shaft could increase the Nu number. Up to %18. Also, the results showed that the rotational shaft could significantly increase the pressure drop and friction factor. The maximum increment of %78 was achieved for friction factor. It was revealed that the use of rotational shaft could be more efficient at low Re numbers and low dimensionless rotational speeds. Also, it was found that by the increment of Reynolds number and being in the transient regime the efficiency of the system would improve.

Analytical modeling of a radiant cooling system for thermal performance analysis

The aim of this paper is to investigate the energetic and exergic performance of the close loop radiant cooling panel system using mathematical modeling. In this study, efforts have been made to determine the COP, cooling capacity and second law efficiency of radiant panel at different operating parameters. Based on results, it is found that mass flow rate of heat carrying fluid plays a major role as compared to plate thickness and pipe thermal conductivity in the thermal performance radiant panel system. The increase in mass flow rate of circulating fluid from 0.01 kg/s to 0.04 kg/s leads to increase the cooling capacity of the radiant panel around 20.9%. Whereas the reduction in inlet temperature of 22% increases the rate of heat transfer to around 62% and the maximum COP is observed as 2.26 at 0.01 kg/s. Further, COP of the panel is found to increase with the increase in plate thickness and with the increase in inlet fluid temperature the exergy of the panel reduces.