Heat Exchange Coefficient Research Articles

Numerical Simulation Calculation of Thermal Discharge Water Diffusion in Coastal Nuclear Power Plants

Coastal areas have become the primary location for nuclear power plant sites due to the convenience of seawater cooling. In China, the diffusion range of thermal discharge water has always been a hot research topic as the basis for sea use areas. The main physical process of heat loss in the numerical simulation of thermal discharge from power plants is heat exchange between the water surface and air. Currently, the methods for calculating this heat loss include the heat flux method and the water surface heat exchange coefficient method. Taking a power plant in Zhejiang as the research object, the MIKE 3 temperature–salinity module (heat flux method) and MIKE ECO Model (water surface heat exchange coefficient method) were used for modeling, and the diffusion of thermal discharge water under different modules was compared and analyzed. The results show that the calculated area of the water temperature rise envelope between the two modules differed significantly under the same conditions. The surface 4 °C temperature rise area, as calculated using ECO Model, was 45.8% smaller than that of the temperature–salinity module. The MIKE 3 temperature–salinity module can simulate the heat accumulation of water under thermal buoyancy, producing a significant thermal stratification phenomenon; the ECO module does not accurately represent the heat exchange process between the water surface and the air, and it does not adequately capture the significant vertical stratification that occurs in real-world scenarios. On this basis, this study not only lays a foundation for further exploring the impact of thermal discharge water from this powerhouse on the structure of surrounding marine biological communities and dissolved oxygen content in water bodies but also provides scientific evidence for the selection of modules when simulating thermal discharge water with the MIKE model.

Convective heat transfer enhancement by corona discharge in a wire–cylinder electrostatic precipitator with the water-cooling system

Aiming at the current wet plume problem, we present a novel technique to integrate electrostatic precipitator (ESP) and heat exchanger in order to simultaneously increase heat exchange efficiency, collect water and control particle emission from flue gases. Unlike the conventional ESP, a lab-scale wire–cylinder type ESP with the collection electrode cooling by water is used for investigations in this paper. Our research indicates that the heat transfer coefficient of the ESP rises with reducing gaseous velocity, increasing applied voltage, corona current or gas temperature. The maximum improvement of the total heat transfer coefficient of 271% was achieved at 0.15 m/s, 80 °C and 16 kV of the negative discharge. Moreover, the presence of Particle matters can enhance the heat transfer coefficient by 9%–16%. The ionic wind, which is quantitatively expressed by electro-hydrodynamic number, plays a key role in modifying the gas flow patterns and consequently improving the heat exchange coefficient. For lowering the overall energy consumption, it is suggested that the ESP should be operated at a specific mode of low voltage and high current.

Modeling the wetting behavior of grinding wheels

Helical flute grinding is an important process step in the manufacturing of cylindrical cemented carbide tools where the use of cooling lubricants is a defining factor determining process performance. Finding optimal parameters and cooling conditions for the efficient use of lubricant is essential in reducing energy consumption and in controlling properties of the boundary zone like residual stresses. Any mathematical model describing the interactions between grinding wheel, lubricant and workpiece during the process has to account for the complex microstructure of the wheel; however, this renders the identification of parameters like slip or heat exchange coefficients numerically prohibitively expensive. In this paper, results from grinding oil droplet experiments are compared with simulation results for the wetting behavior of grinding wheels. More specifically, finite element simulations of the thin-film equation are used to identify slip parameters for different grinding wheel specifications (grain size, bonding structure, wetting status). Our results show that both the bonding and the grain size have an influence on the wetting behavior. The slip parameters that we identified account for the fluid-microstructure interactions and will be used to effectively model those interactions in more complex 3D fluid-dynamic simulations via the Beavers-Joseph condition.

Assessing indoor thermal comfort of rock-cut architecture in Meymand world heritage site during winter and summer

Rock-cut architecture is among the unique types of ancient architecture in Iran, which creates human living space by excavating the stone mass in the rocks. The rock-cut architecture benefits from utilization of the temperature of the groundmass in cold and hot weather by penetrating its heart. The World Heritage Site of rock-cut architecture in Meymand with more than 360 architectural units is considered as an evolved and appropriate example of such architecture in the semi-hot and dry foothill climate of Iran. The present study aims to determine indoor thermal comfort level of some buildings in the site as mentioned earlier to measure their fitness to climatic conditions of the area without energy consumption. To this aim, Predicted Mean Vote (PMV) index was determined for four selected buildings through the indoor bioclimatic monitoring, as well as calculating the level of the above-mentioned index during December–February and June–August.​ Then, the ASHRAE standard questionnaire was prepared to evaluate and compare the comfort sensation feedback of the occupants in the aforementioned buildings with indoor PMV calculation results. The results indicated that the average indoor PMV during the winter and summer equaled -2.17 and -1.07 with a cold and cool sensation, respectively, which is in line with those of the questionnaire in which most occupants reported a neutral and cold sensation during the winter and a cool one during the summer. In addition, calculation of the percentage of proximity to the comfort zone for indoor environment of the above-mentioned buildings showed they are closer to comfort by 22% and 36%, during the winter and summer, respectively Finally, the results indicated the rock-cut architectural buildings had provided conditions by applying the high thickness of their architectural layers, low heat exchange coefficient of the walls, and temperature of the groundmass, which they are in the comfort zone without energy consumption in summer and provide living conditions with minimal energy consumption in winter.

Metal-free design of a multilayered metamaterial with chirped Bragg grating for enhanced radiative cooling.

A wideband, all-dielectric metamaterial structure for enhancing radiative cooling is investigated. The structure is optimized to reflect most of the solar irradiance window (between 0.3 µm-3 µm), which is one of the biggest challenges in highly efficient radiative cooling coatings. The design is based on the principles of Bragg gratings, which constitutes a simple synthesis procedure to make a broadband reflector of reduced dimensions, without metallic layers, while keeping a flat enough response in the entire bandwidth. Numerical results show that reflection of solar irradiation can be easily tailored and maximized using this method, as well as the net cooling power of the device, about ∼79 W/m2 at daytime (about double at night-time) and a temperature reduction of 23 K (assuming no heat exchange) and 7 K assuming a heat exchange coefficient of 10 W/m2/K, for a device and ambient temperatures of 300 K and 303 K, respectively. This occurs even in detriment of absorption in the atmospheric window (8 µm-13 µm). Results also show the importance of efficiently reflecting solar irradiance for such technologies and its relevance in synthesis and design without using metallic components.

Influence of hydrodynamic parameters in plate heat exchangers in ice slurry transport

Ice slurry is applied in different refrigeration and air conditioning systems, and among its main advantages is the ability to store latent heat and be pumpable as cold water. This fluid can be used for thermal storage instead of cold water or solid ice, since, at certain concentrations, it can be pumped through heat exchangers. For this technology to be widely used, the equipment must be designed considering the heat exchange characteristics. This study developed an experimental system to study the parameters that influence heat exchange, using ice slurry in a plate heat exchanger and water as a thermal load. Several situations with different flows and initial fractions of ice in ice slurry were analyzed. The input conditions of the thermal load were kept constant, the temperature, the pressure drop and the flow of the ice slurry in the heat exchanger were controlled, according to the Nusselt, Prandtl, and Reynolds coefficients. Results show that the global heat exchange coefficient increases with the increase of the initial ice fraction; the cooling capacity of the heat exchanger increases when ice slurry is used as a fluid. For flow rates greater than 0.16 kg/s and ice fractions above 11%, ice crystals are generated at the heat exchanger outlet, due to the ice slurry’s higher viscosity, at approximately 0.003 Pa at the outlet, since the pressure drop increases as the initial ice fraction increases. Results show ice slurry thermohydraulic behavior. Its use shows favorable characteristics in heat exchange.

Study of heat retention phenomena by a thermal insulating wall made of tow and plaster in free convection from the analysis of thermal impedance

The thermal behavior of a thermal insulating wall made of tow and plaster is studied by analyzing the behavior of thermal impedance. The evolution in thermal impedance through the wall is shown by highlighting the behavior over time and the effect of the heat exchange coefficient on the front face. SDepending on the climatic stresses on the front face, the analysis shows that the wall stores and releases energy by capacitive effect or by inductive effect. The peaks of the impedance make it possible to evaluate the effective thermal insulation layer for the tow-plaster material according to the external climatic constraints.

The effect of additional external cooling on the formation of the weld during pipe welding.

The paper presents an analytical description of real-time thermal field arising in industrial welding of a pipe wall taking account of possible accelerated cooling of the seam after welding. Considered was the case when the surface opposite to the welding bath is cooled with a spray of air or air-water mixture under certain pressure. A concept of characteristic cooling time is introduced depending on heat exchange coefficient at the cooled surface of the pipe. Calculations were performed for various heat exchange coefficient values ranging from 1•10–3 (no cooling) to 6•104 W/(m2•K) (cooling with air-water mixture under pressure). Results show that even cooling with an air spray results in sufficient increase of cooling rate, and in case of cooling with an air-water mixture it increases much greater. Namely, the time to finish crystallization becomes shorter, and cooling rate in the temperature range of austenite transformation increases. There are a number of factors favouring the greater impact toughness of the welded pipe under accelerated cooling of the internal pipe surface: accelerated crystallization resulting in grain size refinement; accelerated cooling of gamma phase resulting in more disperse ferrite-pearlite (or even bainite) structure, which may have also resulted in an increase of impact toughness; all these structure changes do nevertheless result in martensite formation

The Influence of Refined Urban Morphological Parameters on Dynamical and Thermal Fields in a Single-Layer Urban Canopy Model

In this study, localised and non-uniform urban morphology (UM) and urban fraction (UF) parameters are implemented in a single-layer urban canopy scheme in the Weather Research and Forecasting (WRF) mesoscale meteorological model. The purpose of this research is to evaluate the effect of the refined parameterisation scheme on the simulation of dynamic and thermal fields in the urban canopy of the Guangzhou metropolitan area. The results showed that, compared with the default urban canopy parameters of the WRF model, using the localised UM parameters resulted in the most significant improvement in the 10 m wind speed simulation. In urban districts, the mean bias between the observed and simulated 10 m wind speed was reduced significantly by 59% from 2.63 m/s to 1.09 m/s during the daytime. For the thermal environment simulation during the daytime, higher UF and UM values resulted in lower surface albedos and generated narrower street canyons compared with the default modelling setting, which caused more heat to be trapped in the urban canopy and ultimately led to an increase in the surface skin temperature (TSK) and a largely increased ground heat flux (GRD). As a result, at night, more heat was transferred from the ground to the surface, producing a higher TSK. The effect of the localised UF on the sensible heat flux (HFX) was closely related to the near-surface temperature gradient. The UM caused the HFX to increase during the daytime, which was related to the near-surface heat exchange coefficient in the lower model layers. As the high-resolution UM significantly altered the urban geometry, the dynamic environment simulation resulted in a large increase in friction velocity and a decrease in wind speed.

Experimental Investigation of Enhancement of Natural Convective Heat Transfer in Air Using Ultrasound

One of the methods to improve convective heat exchange is the application of ultrasound assistance. However, little is known about ultrasound application in the air. The main purpose of this study is to investigate the effect of ultrasound on natural convection cooling. The tests are based on the cooling of the metal samples (in four different shapes) preheated to a temperature of 60 °C. Cooling takes place in free convection without and with the use of ultrasound at different powers (50 W, 100 W, 150 W, and 200 W). The study uses a mathematical model based on a small Biot’s number assumption. The values of the convective heat exchange coefficients are determined by using an approximation of the experimental results. The coefficients obtained are an increasing exponential function of the applied ultrasound power. This study indicates the possibility of using ultrasound to improve heat transfer by free convection.

Design of Bi2Te3-based thermoelectric generator in a widely applicable system

A Bi2Te3-based thermoelectric generator (TEG) system cooled by heat radiation fin (HRF) applies to most scenarios of low-temperature power generation and waste heat recovery, while the high performance of the TEG system requires the precise design of the structure. Here we developed a theoretical model to simulate the thermal-electric coupling of the TEG system and analyzed the Bi2Te3-based TEG system with the aid of numerical and experimental methods. Results show that the increase in height of TEG not only improves the conversion efficiency but leads to a peak value of output power, which is much different from the traditional view that the output power solely decreases with the increase of height of TEG. Compared to the thickness of the fin, the height of the fin plays an essential role in optimizing the performance of TEG. By adding an electrical fan to improve the heat exchange coefficient, the net output power is doubled and the net conversion efficiency is improved by more than 80%. Besides, the structure of the TEG system is designed for different material parameters via the theoretical method. We anticipate that our results will guide the design and optimization of the TEG system.

Reducing energy consumption of heat pumps by introducing a thermal energy storage system

We proposed a short-term thermal energy storage system by applying Phase Change Materials (PCMs) in air ducts connected to an indoor air conditioning unit within a ceiling space. PCM tubes were arranged in a staggered layout in the air duct and cooled by chilled air from the air conditioning unit during “charging” mode, whereas the PCM tubes released cold energy to the room space without operation of the compressor during “discharging” mode. The charging mode allowed for an increase in the partial load ratio of the heat pump to prevent low-efficiency operation at <20% load, which can increase seasonal energy efficiency. We constructed an energy simulation program, where the heat exchange coefficient between the PCM tubes and air was varied with airflow rate and the temperature difference between the PCM and air, in accordance with results from CFD analysis. Switching between the charging and discharging modes was based on the outlet temperature (Tout) and 30% of the rated capacity of the indoor unit (Lr). The heat balance between the charging and discharging modes was improved by this change to the system control. A higher load factor and higher COP resulted in a 7.6% reduction in energy consumption.

Numerical Study of Forced Convection Between Two Elliptical Cylinders with Variable ThermoPhysical Properties

This study presents a numerical simulation of the three dimensional laminar forced convection between two elliptical horizontal cylinders with variable thermophysical properties using a program generated in FORTRAN language. The inner cylinder is uniformly heated whereas the outer cylinder is adiabatic. The flow and thermal fields are modeled by the continuity, momentum and energy equations with appropriate initial and boundary conditions using an elliptical coordinate system. The model equations are numerically solved by a finite volume numerical method with second order accurate spatiotemporal discretization. The variations of axial velocity, temperature and Nusselt number have been studied in three cases of forced convection, which are 100, 200 and 300 of Reynolds values. The results obtained show that the change in flow regime with increasing Reynolds number value has a significant effect on the value of Nusselt number, rather the heat exchange coefficient. Where Re=300 gives the best heat exchange.

Cr2O3 Doping Effect on Silica Glass Cooling Rate

In this paper we present the results on the effect of the SiO2 melt cooling rate on the morphology of its surface. The method of atomic force microscopy explores a surface roughness of the silica glass cooled at different rates after its heating up to more than 2100 °C. The study compares three modes of cooling glass rods with a diameter of 14 mm: convective heat exchange with the surrounding air, quenching with dropping the sample into a vessel with water, and radiation of silica glass doped with 0.1 wt% Cr2O3. The surface roughness of silica glass in the second and third modes of cooling was significantly less than in the conditions of convective heat exchange. Doping the silica glass with Cr2O3 increases the heat exchange coefficient by 2 times and decreases the surface roughness. The results of the research can contribute to improving the mechanical and optical properties of silica glass light guides.

Air-water cooling mathematical model in cooling towers with vertical contact grilles

This paper examines the fan cooling towers with vertical contact grilles operation. The water-air cooling process a general mathematical model in cooling towers has been developed. Water-air interaction peculiarities on contact vertical grids have been considered, which allowed to obtain functional dependences for heat and mass exchange coefficients determination taking into account the cooling tower sprinkling devices design peculiarities under consideration.

Study of Energy Storage Phenomena in a Flat Wall Containing a Kapok-Plaster Material in Frequential Dynamic Regime-Influence of Depth

We propose, in this article, the study of the phenomena of energy storage in a wall in frequency dynamic mode. The optimal heat exchange coefficient and the maximum pulsation were determined from the temperature and flux density curves, respectively. An electric-thermal analogy made it possible to determine the phenomena of energy storage from Bode diagrams of thermal capacity and thermal inductance.

Integration of molten carbonate fuel cell and looped multi-stage thermoacoustically-driven cryocooler for electricity and cooling cogeneration

Thermal management is essential for high-temperature molten carbonate fuel cell (MCFC) because the accumulated waste heat may degrade the durability. In this paper, looped multi-stage thermoacoustically-driven cryocooler (LMTC) is proposed to reuse the waste heat from MCFC for cooling production, which not only can tackle with the thermal management issue but also can provide additional usages. Accounting various irreversible dissipation, the models of MCFC, LMTC and MCFC-LMTC hybrid system are analytically formulated. Performance features of MCFC-LMTC hybrid system are revealed and the advantages are expounded via calculation examples. Calculations indicate that the maximum power density and corresponding efficiency of the hybrid system are 1688.9 W m−2 and 39.7%, which are 11.4% and 1.3% bigger than that of the sole MCFC system, respectively. By comparing with other available systems, the superiority of using LMTC to recover MCFC waste heat for refrigeration is clearly demonstrated. Considerable parametric studies show that the heat-transfer coefficient of hot heat exchange for LMTC is not suggested to be greater than 2.5 × 10−3 W m−2 K−1. In addition, an increase in the working temperature, working pressure of MCFC, reactant concentration or engine stage number of LMTC positively benefits the hybrid system performance, while an increase in the thermodynamic loss coefficient worsens the hybrid system performance. The obtained results may offer new insights into improving the performance of MCFCs through thermal management approaches.

Spatializing the roughness length of heterogeneous urban surfaces to improve the WRF simulation-Part 2: Impacts on the thermodynamic environment

Roughness length (Z0) is a fundamental parameter in atmospheric boundary layer theory that affects the transfer of momentum and heat. However, this parameter in urban areas in the WRF model is typically assigned a fixed value. In a previous study, we developed a Z0 database for the heterogeneous urban underlying surface of Guangzhou and demonstrated that its application in the WRF model had significantly improved the simulations of wind speed. In this study, the impact of Z0 update on the dynamic and thermal environment in Guangzhou urban areas is investigated through two numerical experiments: the base experiment applies the model-default Z0 value (0.5 m) for the urban areas and the sensitivity one uses a heterogeneous Z0 map (with higher Z0 values in most urban areas) estimated in the previous study. The results show that the temperature and relative humidity at 2 m height are not sensitive to this Z0 modification. For the dynamic environment, the modification of Z0 decreases the wind speed at 10 m height and increases the friction velocity. The reduction of horizontal wind speed due to Z0 increases reaches heights of several hundred meters, especially below a height of 400 m (950 hPa). Additionally, the updated Z0 leads to an increase in the turbulent momentum exchange coefficient over the urban underlying surface, resulting in a more vigorous turbulent mixing and elevated planetary boundary layer height. For the thermal environment, the Z0 increases result in a higher turbulent heat exchange coefficient near the surface layer, which contributes to the vertical heat flux transport. Moreover, the Z0 increase enhances the sensible heat flux during daytime and reduces the land surface temperature and ground heat flux in Guangzhou central urban area. Due to the reduced heat storage during the daytime, the amount of heat released at night decreases accordingly, thus causing a reduction in the land surface temperature at night and a decrease in the sensible heat flux. This study provides a comprehensive understanding of the sensitivity of the thermodynamic environment to the urban Z0 increase and reconfirms the significance of a realistic Z0 dataset in atmospheric modeling.

Statistical learning model for fuel-cladding heat exchange coefficient evaluation in SFR

Multiphysics modeling has become a central point in nuclear reactor simulation as it takes into account interactions between physical fields involved. Nuclear reactors are highly multi-physics systems as neutronics, thermal transfer, mechanics and hydraulics are interacting to produce and maintain the power. In this system, numerous issues come from material behavior. In the case of Control Rod Withdrawal (CRW) accident the main issues come with fuel behavior as the major risk to prevent is the fuel melting and spread in the core, leading to its partial or complete meltdown. In these situations, specialized codes are often used to predict properties and the state of the fuel by aggregating isolated models, each one corresponding to a single phenomenon. This type of approach is very efficient for understanding global sequence of the accident and predicting numerous physical variables of the problem. However it is often very expensive in calculation time. In this paper we focus on developing a fast tool based on machine learning models in order to speed up the calculation of specific variable of interest. We propose here a brief physical context of the study, a description of the method used to chose and to train models and finally an evaluation of models. Using this tool in a multiphysics scheme will add negligible penalty on calculation time. The model is focused on modeling fuel cladding heat exchange coefficient hgap based on data extracted from GERMINAL-V2 fuel performance code developed at CEA for SFR. Considering the strong non-linearity of the variation of the hgap, it appears that Random Forest models and AdaBoost present the smallest deviation, lower than 1% and the faster response, less than 10μs. However, the sensitivity of the hgap for highest burnups does not allow to get a metamodel with a deviation smaller than 20% for the application case.

Experimental determination of the heat exchange coefficient of industrial steam pipelines

The article discusses and analyses the factors related to the use of a thermal imaging camera to determine heat loss in industrial steam pipelines at factories from chemical and metallurgical industry, by measuring their surface temperature. The generally accepted enthalpy method for determination of the loss has serious drawback it gives accurate results, but in averaged units in which it is impossible to take account of the contribution of the different parts and components of the pipeline in the total heat loss. The unavailability of information on where, how and in what way along the route this loss is formed does not allow prompt and specific measures to be taken for its reduction. An attempt has been made to structure empirically a reliable analytic dependence for determination of the heat exchange coefficient, bringing together the various factors influencing the heat exchange. By the method of the least squares the free coefficient and the exponent have been determined of criterion equation satisfying initial and boundary conditions of the experiment. Based on the obtained results for determining the heat losses by measuring the surface temperature of steam pipelines with a thermal imaging system, a reliable and acceptable method is proposed, which has a place in engineering practice. For this purpose, an industrial experiment has been carried out at three actually operating steam pipelines of different diameters and steam parameters. A criterion equation has been derived that can be used as a mathematical model for software products with a practical orientation for regular assessment of heat losses of steam pipelines. Values of heat losses determined through energy balance of heat carrier and heat flux from the outer surface of the steam pipelines have been compared. Results for the heat exchange coefficient, obtained through a balance have been compared with the analytically determined values based on current standards. A new method has been developed for express evaluations of the current heat losses of the steam pipeline in real time, as the sum of the losses through its individual components gives as average values 9÷12 % increased results for the losses compared to the enthalpy method. Its great advantage is that it can be used selectively to determine the losses through individual sections of the steam pipeline.