Experimental investigation in improving thermal performance of passive heat removal system using mist assisted evaporative cooling

Demands to increase the safety and reliability of the modern nuclear reactors are constantly being made. Typical demands include the increases the thermal performance of the passive heat removal system PHRS by removing larger amounts of energy and applied a technique that leads to reduce size and weight of the PHRS unit. This article presents an experimental investigation on the natural convection based on alternative cooling approach using water mist. The results obtained from the related experimental work indicated that the heat transfer rate was enhanced over that for the airflow as a result of water mist evaporation on the surface of the tubes. The Nu number increases by about 148%, 144%, and 128% respectively for all tube rows compared with airflow. The experimental results of the thermal performance factor values obtained by suspended water mist were correlated as functions of Rayleigh number (Ra) and Weber number (We). Subsequently, the predicted thermal performance factor from the correlation was plotted to compare with the experimental data. It was found that the thermal performance factor was within ±11%. © 2020 PENERBIT AKADEMIA BARU

Numerical simulation and experimental investigation of heat transfer and flow structures around heated spherical bluff bodies

The objective of this work is to evaluate the influence of vortices on heat transfer behaviour and a flow structure around a heated sphere. Numerical simulation and experimental verification are performed using a stationary copper sphere located inside a cylindrical channel with a constant channel-to-sphere diameter ratio. Numerical simulation is done for three-dimensional steady-state flow using ANSYS-FLUENT by solving the Reynolds-averaged Navier Stokes (RANS) equations. Over the test range of Reynolds numbers (2500-55000), CFD simulation results are in reasonable agreement with experimental data. The importance of vortices on heat transfer behaviour was investigated by taking the surface temperature and heat transfer coefficient (HTC) measurements around the sphere surface as a function of a zenith angle. The CFD simulation results confirmed that the impact of vortices on heat transfer behavior occurred in a lower-rear area of the sphere with a zenith angle (from 120° to 180°). © Published under licence by IOP Publishing Ltd

An experimental investigation on the transient heat transfer characteristics using air/water droplets two-phase flow

The present study focused on evaluating the heat transfer behavior and predicting the surface resulting status during air/water droplets two-phase flow. Transient heat transfer based on the lumped capacitance model (LCM) was investigated experimentally under a range of water droplets concentration, surface temperature, and varying Re number. Compared with a single-phase air cooling, the transient surface temperature decreased with the increase in water droplets concentration and Re number. At the same cooling time, the surface temperature decreases about 13.5%, 47%, and 53.2% for (j = 46.79 - 111.68 kg/m2 hr). It was also noticed that the heat transfer coefficient increased with the increase in water droplets concentration and reach its maximum value at (j = 111.68 kg/m2 hr). Based on the analysis of the experimental results, the heat transfer mechanism due to the impacting of water droplets on the sphere surface was classified into three important physical regimes. Clear convection heat transfer regime corresponds to the dry region (region I); Convection and evaporation regimes correspond to the dry-out and wet regions (region II and III). © 2020 Institute of Physics Publishing. All rights reserved

Investigation of heat transfer coefficient of spherical element using infrared thermography (IR) and gas - Water droplets (mist) as working medium

The changes that do occur in the air ambient properties with increasing concentration of water mist (fog) have a significant influence on heat exchange process between the building and the surrounding medium. In the present paper, the average and local heat transfer coefficient was estimated from surface temperature data obtained using infrared (IR) thermography. In particular, the experiments were performed on steady-state conditions under constant heat flux for a single sphere suspended in cylindrical channel using air as well as air / water droplets as working fluid. The influence of the different factors such as Re numbers and water flux density on heat transfer behaviors are examined. Five cases are tested under range of water flux density (0 -111.68 kg m-2 hr-1). The experimental results confirmed that the heat transfer coefficient significantly increased with increase in water flux density. The heat transfer coefficients are respectively 1%, 19.7%, 90.2% and 134% higher than those in air-cooling. The results also revealed that infrared (IR) thermography it has proven to be very efficient in measuring the surface temperature distribution. The results obtained by infrared thermography (IR) are compared with calibrated thermocouples and the temperature distribution is found to be in close agreement with 2.94% average error. © 2019 Published under licence by IOP Publishing Ltd

Heat transfer intensification in emergency cooling heat exchanger of nuclear power plant using air-water mist flow

Advanced nuclear power plants are equipped with passive systems for emergency decay heat removal from reactor equipment (PEHRSs) in case of development of accidents accompanied with primary cooling circuit leakage and for transferring heat to the final heat absorber (ambient air). Here, intensity of heat dissipation to air from the heat exchanger outer surface achieved by buoyance induced natural convection is extremely low, which necessitates the need to expand heat conductivity surfaces and to apply different types of heat transfer intensifiers (grooves, ribs and extended surfaces, positioning at higher altitudes, etc.). Intensity of heat removal is also strongly dependent on the ambient air temperature (disposable temperature head). Construction of nuclear power plants in countries with high ambient temperatures (Iran, Bangladesh, Egypt, Saudi Arabia and others) with characteristic high level of ambient air temperature imposes additional requirements on the expansion of heat exchange surfaces. Results of experimental investigation of intensification of heat exchange by low energy-intensity ultrasound supply of super-small liquid droplets (size ~3 mm) in the cooling air are provided in the present paper. In such case, transfer of heat between the cooled surface and cooling airflow involves the following three physical effects: convection, conductive heat exchange and evaporation of water droplets. The latter two effects weakly depend on the ambient air temperature and ensure active heat removal in any type of situation. Investigation was performed using high-precision calorimeter with controlled rate of heat supply (between 7800 and 12831 W/m2) imitating heated surface within the range of Reynolds numbers from 2500 to 55000 and liquid (water) flow rates from 23.39 to 111.68 kg/m2·h-1. The studies demonstrated that presence of finely dispersed water results in significant increase of heat transfer compared with the case of application of purely air-cooling. With fixed heat flow energy efficiency increases with increasing concentration of water reaching the values in excess of 600 W/m2·degree-1, which is 2.8 times higher than for the case of air-cooling. Application of the suggested technology for intensification of heat exchange in dry cooling towers of nuclear and thermal power plants used in the conditions of hot and extreme continental climate is possible subject to further investigation for the purpose of specification of optimal ranges of heat exchange intensification. © 2019 Obninsk Institute for Nuclear Power Engineering, National Research Nuclear University 'MEPhI'. All rights reserved