Non-uniform Heat Flux Research Articles

Stability and thermal analysis of a single-phase natural circulation looped parabolic trough receiver

The sustainability and reliability of solar parabolic trough technology can be enhanced by introducing a natural circulation loop. The configuration that suits better is horizontal heater horizontal cooler, which is the most efficient but least stable loop. Hence, as an effort towards the evolution of natural circulation looped collector, 3D computational analysis of a 70 mm uniform diameter square loop (Dowtherm A as the heat transfer fluid) is done with uniform and non-uniform heat flux (SolTrace based) as well as different instability restraining techniques. Prior to the analyses, the stability maps were generated and validated, which predicts high level of instability. Later, the validated computational model is used for parametric analyses, which includes heat flux pattern, loop tilt, use of orifice, and combination of tilt and orifice. Since the loop geometry is highly unstable (ratio of loop length to diameter is 57), the stable operation is observed only with high tilt (θ = 45°), low beta ratio (β = 0.25) and combinations of tilt and orifice (β (0.50) + θ (15°) and β (0.50) + θ (30°)). However, the latter method is suggested for trough application as the former two versions are impractical. Further, the heat load analysis with uniform and non-uniform flux designated insignificant variation in system performance. Since the loop is effective with restraining techniques and its performance is on par with the conventional trough system (predicted by the analytical model), an effort towards implementation of natural circulation loop in parabolic trough technology is prolific.

Numerical study on preheating process of molten salt tower receiver in windy conditions

In the preheating process of the molten salt tower receiver, the thermal stress is large and the over-temperature problem is prone to occur. The finite volume method and two-dimensional thermoelastic method are combined in this paper to study the preheating process of the receiver tube in windy conditions. The experimental results of a lab-scale receiver verify the numerical model. Then, the influence of heat flux, wind speed, wind direction, ambient temperature and uniformity of heat flux distribution on the preheating process is explored. Finally, the influence of salt filling temperature and salt filling mass flow on the salt circulating is also revealed. The results indicate that the increase in heat flux, decrease in wind speed and increase in ambient temperature decrease the preheating time. The wind in the direction of 30° from the front side of the tube has the greatest influence on the preheating process. Compared with the no-wind condition, the preheating time increases by 177.5%. The maximum tube wall temperature and thermal stress are significantly higher under non-uniform heat flux distribution than those under uniform heat flux distribution. The salt inlet temperature and salt inlet mass flow respectively affect the stable value and reduction rate of thermal stress.

Thermal, thermodynamic and exergoeconomic investigation of a parabolic trough collector utilizing nanofluids

The exploitation of solar energy facilitates the renewable energy paradigm. In this regard, parabolic trough collectors (PTC) are considered as a useful set-up to absorb solar energy. Simultaneous study of thermal, thermodynamic, and exergoeconomic performance of PTC systems paves the way for designers and manufacturers to not only have a better insight into understanding the underlying concepts about the operation of PTC systems but also to find the most effective and cost-effective circumstances. This study aims at analyzing a practical PTC system by considering an evacuated absorber tube with glass cover, non-uniform heat flux, and taking into account the convective and radiative heat losses. Obtained results demonstrate that employing the glass cover, especially in the low Reynolds numbers regime could remarkably reduce the heat losses up to 22%. The present analysis indicated that considering oil temperature-dependent properties reduced the friction factor around 94 % in the low Reynolds numbers regime compared to that of the cases based on constant properties. In this work a CFD code in the OpenFOAM software was developed to simulate both laminar and turbulent regimes with Lien cubic k-∊ model (non-linear eddy viscosity model) by adding three types of nanoparticles (Al2O3,Cu,andSWCNT) individually into the synthetic oil. Moreover, the Buongiorno’s model (BGM) which considers Brownian, thermophoresis, and turbulence diffusion phenomena was implemented to model the nanoparticles and base fluid interactions. It was found that adding nanoparticles and increasing the Reynolds number have no substantial impact on thermal efficiency of the system, whereas an optimum Reynolds number was found for exergy efficiency and net profit per unit transferred heat load (ηp) of the system. Although injecting SWCNT into pure oil led to augmentation of average convective heat transfer coefficient, PEC, and exergy efficiency, it was found not to be cost-effective. It was established that the obtained results by homogenous model (single-phase model) and BGM were roughly the same, with relative difference of less than 3.1 and 2.4% for the average Nusselt number and friction factor, respectively.

Influence of transient heat flux on boiling flow pattern in a straight microchannel applied in concentrator photovoltaic systems

Concentrated photovoltaic (CPV) overheating due to the high radiation intensity became the critical challenge to preventing its benefits in advancing the photoelectric efficiency. To promote the application of CPV, efficient heat dissipation becomes the key technology. Meanwhile, microchannel flow boiling, as one of the most promising solution to advance cooling of high-flux equipment, has attracted increasing attentions in recent years. However, the fluctuating and non-uniform heat fluxes of CPVs may lead to challenges to microchannel boiling for cooling CPV. Therefore, the transient flow and phase change phenomenon in a microchannel with deionized (DI) water was studied through a numerical modeling on flow boiling. The effects of different sudden heat flux increases on flow boiling two-phase flow pattern were analyzed. It was found that different heat flux increase resulted to different flow pattern evolutions, a much higher sudden increase in heat flux leaded to serious variation of flow pattern, especially the fluctuation of elongated restricted bubbles, which may evolve to annular flow or even dry-out. The vapor phase volume fraction showed a much higher growth rate. When the heat flux increase was raised from 97.96 kW/m2 to 580 kW/m2, the maximum vapor phase volume fraction was increased from 0.104 to 0.8064, and the bubble length increased from smaller than 0.3 mm to 1.5 mm in the dry zone, indicating rapid phase change. Based on analysis on pressure drop and vapor volume fraction variation, the period after receiving sudden heat flux was divided into three stages: initial period, rapid development period, and quasi-stable period. With the rising of heat flux increase from 97.96 to 580 kW/m2, the duration of rapid development period was increased from 0.03 s to 0.066 s, the pressure drop increase was obviously enlarged, from 2.43 kPa to 14.51 kPa, and the vapor phase volume fraction increase in the microchannel was raised by 3.3 times to 25.6 times, indicating more intensive boiling, and deeper effect of higher heat flux increase on the phase change. In addition, the fluctuation intensity of pressure drop in the quasi-stable period was obviously increased from 0.025 to 0.193, demonstrating the advancement in flow boiling instability under higher heat flux increase, which should be controlled.

Self-adaptive cooling of chips with unevenly distributed high heat fluxes

• A self-adaptive chip cooling method for high heat fluxes is proposed. • Unevenly distributed high heat fluxes of up to 460 W/cm 2 are effectively removed. • The maximum surface temperature drop amounts to 22.6 K with reduced flow rates. • Coolant flow adapts to local heat fluxes and movable hotspots are cooled effectively. The heat load scenario of a real chip is unsteady and spatially variable and gives rise to regions of concentrated high power. Conventional liquid cooling techniques are always inadequate to tailor the flow distribution to the changing thermal environment and cool the resulting high-heat-flux regions effectively. In this work, the self-adaptive cooling for unevenly distributed high heat fluxes is proposed. It contains a matrix of microfluidic units with integrated thermal-sensitive hydrogel valves. The units are independently controlled and deliver the coolant to where it is needed. A numerical model is developed to investigate the flow and temperature distribution of the cooling unit. By introducing the self-adaptive mechanism, the near-wall flow is enhanced remarkably. Nonuniform heat fluxes of up to 460 W/cm 2 are effectively dissipated by means of the developed self-adaptive cooling method. The maximum surface temperature drop amounts to 22.6 K, meanwhile the total flow rate can be reduced by nearly an order of magnitude. The microfluidic unit array adapts to local heat fluxes by reconfiguring its internal flow-channel structure automatically and thus movable hotspots are elaborately cooled. The self-adaptive cooling scheme offers both effective and smart thermal management for modern electronic devices.

Impact of non-uniform heat flux on ventilation performance of solar chimney

Solar chimney (SC) is an effectively practical way to enhance building natural ventilation through thermal buoyancy force, which is induced due to the temperature difference between indoor and outdoor air temperatures. The ventilation performance of solar chimney directly depends on the heat flux distributions and values in the absorbed plate. Recently, many researches have been done on the thermal performance of solar chimneys under uniform heat flux. However, the uniform heat flow distribution cannot reflect the uneven characteristics of the solar radiation. There are few studies focused on the flow characteristic and thermal performance of solar chimneys under non-uniform heat flux distributions. In this paper, a two-dimensional numerical model of solar chimney integrated building is firstly established to explore the natural ventilation performance under non-uniform heat flux distributions. The numerical simulation is conducted to predict the flow and heat transfer characteristics in the room as well as in solar chimney. Subsequently, the effects of the location of the vent are analyzed to evaluate the performance of the solar chimney system. Finally, a comparison between the ventilation performance of solar chimney under the uniform heat flux and the non-uniform heat flux distributions is presented, and the thermal and flow structures of the solar chimney integrated building are further illustrated simultaneously. Research results show that the ventilation performance of solar chimney is closely related to its heat flux distribution, and the inhomogeneity of the heat flux of solar chimney will decrease the ventilation performance of the building. In addition, the position of the vent have highly influence on the flow structure and thermal performance for the solar chimney as well as the building. This paper will provide theoretical basis and technical approach for the improvement of the performance of the solar chimney, especially for the nonuniform heat flux distribution.

Coupling Effect between Heat Flux Distribution and Buoyancy of Supercritical Co2 Heat Transfer with Nonuniform Heat Flux in Parabolic-Trough Collector

Coupling Effect between Heat Flux Distribution and Buoyancy of Supercritical Co2 Heat Transfer with Nonuniform Heat Flux in Parabolic-Trough Collector

Universal Approach to Predicting Full-Range Post-Dryout Heat Transfer Under Uniform and Non-Uniform Axial Heat Fluxes

Universal Approach to Predicting Full-Range Post-Dryout Heat Transfer Under Uniform and Non-Uniform Axial Heat Fluxes

HYDROCARBON FUEL IN HORIZONTAL PARALLEL CHANNELS WITH NONUNIFORM HEAT FLUX BOUNDARY

HYDROCARBON FUEL IN HORIZONTAL PARALLEL CHANNELS WITH NONUNIFORM HEAT FLUX BOUNDARY

Influence of furnace temperature and non-uniform heat flux density on direct reduction process of newly designed carbon containing pellet

Influence of furnace temperature and non-uniform heat flux density on direct reduction process of newly designed carbon containing pellet

Development of a dimensionless rod-bundle CHF correlation based on stepwise regression method, Part II: determination of correction factors accounting for the effect of unheated guide tube and non-uniform axial heat flux profile

Correlation for rod-bundle critical heat flux (CHF) is of vital importance for design and assessment of a Pressurized Water Reactor (PWR). In our previous study, the basic form of a dimensionless rod-bundle CHF correlation has been developed based on stepwise regression with CHF data obtained in test bundles of uniform axial heat flux. It proves that stepwise regression is an effective method to develop simple, but reliable CHF correlation consisting of only dimensionless parameters with rigorous physical meanings. As a continuation of the previous investigation, revision of the basic form was conducted. Dimensionless correction factors accounting for the cold-wall effect due to unheated guide tube and the effect of non-uniform axial heat flux were developed. For the data set of 486 rod-bundle CHF points obtained in 7 test bundles, the average ratio of the measured to predicted CHF is 1.002 with a standard deviation from the mean of 5.51%.

Tailoring the fluid flow distribution in a parallel mini-channel heat sink under multiple-peak heat flux

Parallel micro/mini-channel heat sinks are widely used for the efficient cooling of electronic devices to avoid functional damage and lifetime shortening due to overheating. This study addresses the optimization of fluid flow distribution in parallel mini-channel heat sinks subjected to a non-uniform multiple-peak heat flux to eliminate the temperature hotspots. A 3D heat sink comprising of 16 parallel straight mini-channels is used as a model for study, each mini-channel having the dimension of 1 mm in width, 2 mm in height and 34 mm in length. In particular, an original optimization algorithm is developed to adjust the inlets of these mini-channels according to the temperature distribution on the heating base surface. The fluid flow distribution is thereby tailored, leading to the reduced peak temperature on the heating surface. The effectiveness and robustness of the optimization algorithm are tested and discussed.Results obtained show that the maximum temperature can be reduced by 10 K and 7 K for two-peak and five-peak heat flux cases, respectively, by using the proposed optimization method. The heat sink configuration with optimized channel inlets could always provide smaller thermal resistance than that of the equal channel inlet configuration under different average heat flux or total mass flow-rate conditions. At the same pressure drop, tailoring the flow distribution of the cooling fluid is more effective in reducing the thermal resistance than simply increasing the mass flow rate of the cooling liquid. This optimization method could also be generalized as an efficient thermal management technology for electronic cooling.

The effect of circumferentially non-uniform heat flux on flow boiling heat transfer in a horizontal tube

Flow boiling is an important energy transfer mechanism in several thermodynamic power cycles. In direct steam generation concentrated solar power applications, flow boiling occurs under non-uniform heat flux conditions, such as in horizontal tubes which are predominantly heated from below. This can influence the local and average heat transfer coefficient. In this experimental study the heat transfer characteristics of saturated flow boiling under different circumferentially non-uniform heat fluxes were determined. To demonstrate the influence of a non-uniform heat flux, tests were conducted on R245fa in an 8.5 mm inner diameter tube at a mass fluxes of 200 kg/m²s and 300 kg/m²s, at saturation temperatures of 35 °C and 40 °C, for vapour qualities between 0.2 and 0.7, and for an averaged heat flux of approximately 6.8 kW/m2. Eight non-uniform heat flux distributions, including variations of heating from above, from the side and below, were considered and their results were compared against those of a uniform heat flux distribution. In all cases, the same or comparable total heat rate was maintained. It was found that uniform heat flux conditions exhibited the highest inner wall heat transfer coefficients, irrespective of the mass flux, saturation temperature or vapour quality under consideration. Compared to the uniform heat flux cases, non-uniform heat flux conditions exhibited a significantly reduced average heat transfer which became worse at higher vapour qualities. Depending on the heat flux scenario, heat transfer performances reduced by up to approximately 45%, and for conditions where heating was applied from below, a reduction of approximately 25% was observed. Based on the circumferential local heat transfer coefficients, that were solved via an inverse problem model, it was concluded that the applied heat flux distribution needs more attention and research. The overall pressure drop was not affected by the heat flux condition.

Integrated optical-thermal-electrical modeling of compound parabolic concentrator based photovoltaic-thermal system

The main aim of this work is to propose an integrated optical, thermal and electrical model to obtain the overall performance of a concentrated photovoltaic-thermal system (CPV/T). Monte Carlo ray-tracing (MCRT) simulations are done to obtain the flux profile on the absorber of a compound parabolic concentrator (CPC) for various angles of incidence. The obtained heat flux is mapped onto the solar cell in the finite volume method (FVM) to obtain its temperature profile. The modelling is done for both glazed and unglazed compound parabolic concentrator-photovoltaic/thermal (CPC-PV/T) systems using uniform (average heat flux) and non-uniform heat flux distribution. The obtained numerical results are compared with experimental results available in the literature. The results show that the prediction is accurate, with a maximum deviation in solar cell temperature of less than 3%. The deviations obtained when using non-uniform heat flux are much less than when using average heat flux. The temperature contours show that the temperature profile of the photovoltaic module obtained by using local non-uniform heat flux is different from the temperature profile obtained by using average heat flux, and these results will have a greater impact when designing a cooling system.The absorbed radiation from the optical analysis and average cell temperature from the thermal analysis are given to the five parameter electrical model of a solar photovoltaic cell to obtain the electrical power output. With the results of the integrated optical-thermal and electrical model, the overall useful power from both glazed and unglazed CPC-PV/T systems is calculated. The results show that the performance of the unglazed CPC-PV/T system is superior to the glazed CPC-PV/T system in terms of electrical and overall system output. This work demonstrates the need for a local heat flux profile and a coupled multi-physics model to accurately predict the performance of a concentrated photovoltaic/thermal (CPV/T) system.

Modelling a Calcium-Looping Fluidised Bed Calcination Reactor with Solar-Driven Heat Flux

A new unidimensional computational model is developed to simulate a calcination reactor in a Calcium-looping process for thermochemical energy storage in concentrating solar power systems. The proposed reactor is an absorber tube exposed to concentrated solar radiation. This tube is also the riser of a circulating fluidised bed where the calcination reaction takes place. The proposed heat transfer process models are based on the core-annulus model and the hydrodynamic model is a modified version of the Kunii-Levenspiel model. The model considers the change in the mass flow rate of species and the density change of the phases in the axial direction of the reactor, usually considered constant in the models found in the literature. A higher calcination efficiency, up to 8 p.p., is obtained for the studied reference case when assuming constant density and mass flow rate. Simulations were performed by imposing a solar-driven non-uniform heat flux distribution on the reactor wall. The results show that a 6 m height reactor allows achieving a calcination efficiency of 66% for the reference conditions used. A sensitivity analysis shows that the solids mass flow rate and the inlet bed temperature are the parameters that most affect the calcination process efficiency.

Wind-induced convective heat loss of cylindrical receiver considering the effect of dish concentrator

A numerical model by coupling Monte Carlo Ray Tracing (MCRT) and Computational Fluid Dynamics (CFD) was built to explore the convective heat loss of a cylindrical receiver with built-in coiled tube. Noted that the non-uniform heat flux boundary of the receiver was obtained by optical simulation and the effect of existence of the dish concentrator was taken into account. The variation of flow field and convective heat loss with inclination α, wind direction φ, wind velocity V were analyzed in detail. The results indicate that the presence of concentrator in a windy environment can significantly interfere with the air flow thus affecting the convective heat loss of receiver, and such effect gradually weakens with the increase of α. Except for the case of φ = -90° and 0°≤α ≤ 17°, the convective heat loss with concentrator is always less than that without concentrator. The effect of φ on convective heat loss is complicated and has a great relationship with both V and α. In most cases except for φ = 0°, the convective heat loss takes on a trend of first decreasing and then increasing with varying α. Additionally, a Nusselt number correlation for predicting the convective heat loss of receiver under the effect of dish concentrator was proposed.

Ceramic Insulated Slug Calorimeter for High- Enthalpy Flows

This work presents an approach to the design, and successful experimental validation, of a slug-type calorimeter for long-duration high-enthalpy flows. Specifically, a ceramic shell is used to insulate the slug. Although slug calorimeters have been widely used in arcjets for decades, significant uncertainties are known to be associated with these instruments, which directly affect the uncertainty of critical derived flow quantities such as specific enthalpy. A detailed analysis of the newly proposed design approach is presented. Particular attention has been devoted to the minimization of heat losses from the slug to the shroud, one of the major contributors to the instrument’s uncertainty. The effect of the shroud geometry, insulation thickness, and slug dimensions have been investigated numerically; variable thermophysical properties, nonuniform heat fluxes, and radiation were considered. The analysis showed that the shroud geometry plays a significant role in the heat losses. Based on this design approach, a calorimeter was manufactured to be used in the Office of Naval Research (ONR)–University of Texas at Arlington (UTA) arc-heated wind tunnel “Leste” facility. Experimental measurements in flows with bulk enthalpies up to 4 MJ/kg showed good agreement with the numerical simulations, and measured heat losses were well below the American Society for Testing and Materials standard recommendation.

Impact of thermal radiation and non-uniform heat flux on MHD hybrid nanofluid along a stretching cylinder

The current research investigates the thermal radiations and non-uniform heat flux impacts on magnetohydrodynamic hybrid nanofluid (CuO-Fe2O3/H2O) flow along a stretching cylinder, which is the main aim of this study. The velocity slip conditions have been invoked to investigate the slippage phenomenon on the flow. The impact of induced magnetic field with the assumption of low Reynolds number is imperceptible. Through the use of appropriate non-dimensional parameters and similarity transformations, the ruling PDE’s (partial differential equations) are reduced to set of ODE’s (ordinary differential equations), which are then numerically solved using Adams–Bashforth Predictor–Corrector method. Velocity and temperature fields with distinct physical parameters are investigated and explored graphically. The main observations about the hybrid nanofluid and non-uniform heat flux are analyzed graphically. A decrease in the velocity of the fluid is noted with addition of Hybrid nanofluid particles while temperature of the fluid increases by adding the CuO-Fe2O3 particles to the base fluid. Also, velocity of the fluid decreases when we incorporate the effects of magnetic field and slip. Raise in curvature parameter γ caused enhancement of velocity and temperature fields at a distance from the cylinder but displays opposite behavior nearby the surface of cylinder. The existence of heat generation and absorption for both mass dependent and time dependent parameters increases the temperature of the fluid.

Optimization of Microjet Location Using Surrogate Model Coupled with Particle Swarm Optimization Algorithm

This study aimed to present the design methodology of microjet heat sinks with unequal jet spacing, using a machine learning technique which alleviates hot spots in heat sinks with non-uniform heat flux conditions. Latin hypercube sampling was used to obtain 30 design sample points on which three-dimensional Computational Fluid Dynamics (CFD) solutions were calculated, which were used to train the machine learning model. Radial Basis Neural Network (RBNN) was used as a surrogate model coupled with Particle Swarm Optimization (PSO) to obtain the optimized location of jets. The RBNN provides continuous space for searching the optimum values. At the predicted optimum values from the coupled model, the CFD solution was calculated for comparison. The percentage error for the target function was 0.56%, whereas for the accompanied function it was 1.3%. The coupled algorithm has variable inputs at user discretion, including gaussian spread, number of search particles, and number of iterations. The sensitivity of each variable was obtained. Analysis of Variance (ANOVA) was performed to investigate the effect of the input variable on thermal resistance. ANOVA results revealed that gaussian spread is the dominant variable affecting the thermal resistance.

Analysis of the direct steam generation, thermal stresses, and deformations in a tubular receiver of a solar tower

In solar tower systems, the thermal receiver is a crucial element. Solar receivers may be exposed to high non-uniform heat fluxes, which requires consideration during their structural design. This paper presents a numerical study of the direct steam generation, thermal stress, and total deformations on a representative tube of an external solar receiver. A non-uniform distribution of solar heat flux on the receiver was considered. The sub-cooled boiling and the direct steam generation were analyzed in terms of the temperature field, the volumetric vapor fraction distribution, and steam quality. Under this configuration, the system can produce a steam quality of 0.113%. To develop the structural analysis of the tube, the internal and external temperature fields of its walls were introduced as boundary conditions. Numerical methodologies for direct steam generation, temperatures, and deformation were validated with experimental data. Since the base case presented high deformation (78.63 cm), the use of clips was proposed. Besides, the number, size, and location of the clips were tested. It was found that deformation can be reduced by about 99.1% by placing clips at different positions along the tube. However, the von-Mises stresses increased, but they did not reach the ultimate tensile strength.