Advanced nuclear power plants are equipped with passive emergency heat removal systems (PEHRS) for removing the decay heat from reactor equipment in accidents accompanied by primary circuit leakage to the final heat absorber (ambient air). Herein, the intensity of heat dissipation to air from the outer surface of the heat exchanger achieved by buoyancy induced natural convection is extremely low, which need to a large heat exchanger surface area, apply different types of heat transfer intensification including (grooves, ribs and extended surfaces, positioning at higher altitudes, etc.). The 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) which are characterized by a high level of ambient temperature imposes additional requirements on the increase of the heat exchange surfaces.

The experimental investigation results of heat transfer intensification by a low energy ultrasonic which supply a fine liquid droplet (size ~3 µm) in the cooling air are presented in the present paper. In such case, the heat transfer between the surface and cooling flow involves the following three physical effects: convection, conductive heat transfer, and evaporation of water droplets. The last two effects weakly depend on the ambient air temperature and provide an active heat removal in any situation.

The investigation was performed using a high-precision calorimeter with a 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·m^-2·h^-1.

The studies demonstrated that the presence of finely dispersed water results in a significant increase in heat transfer compared with the case of using purely air-cooling. With a fixed heat flux, the energy efficiency increases with increasing water concentration, reaching the values over 600 W·m^-2·C^-1 at 111.68 kg·m^-2·h^-1, which is 2.8 times higher than for air cooling. With further development of research in order to clarify the optimal areas of intensification, it is possible to use this technology to intensify heat transfer to the air in dry cooling towers of nuclear power plants and thermal power plants used in hot and extreme continental climates.

Nuclear power plants, aerosol cooling, air-water mist, water concentration, heat exchange intensification.

Advanced nuclear power plants are equipped with passive systems for emergency decay heat removal from reactor equipment (PEHRSs) to the final heat absorber (ambient air) in case of development of accidents accompanied with primary cooling circuit leakage and for transferring heat to the final heat absorber (ambient air) (Mousavian et al. 2004; Maio Vitale et al. 2012; Zhang et al. 2012; Dmitriev et al. 2013). For nuclear power plants designed in Russia, passive safety systems were firstly used in the AES-92 Project followed by all subsequent design projects (AES-2006, VVER-1200, VVER-TOI). Design of passive safety system is based on the use of special loop transferring heat to air heat exchangers (Zvirin 1981; Andrushechko et al. 2010; Safety assessment report 2013). PEHRS in its present form consists of four independent loops incorporating heat exchange equipment with air ducts (Galiev et al. 2017). Removal of heat from outer surfaces of the heat exchanger is achieved using the processes based on the action of natural forces, such as unforced natural circulation. Driving forces of the process of natural circulation are extremely weak, which necessitates the development of equipment with expanded heat exchange surface, application of different methods for intensification of heat transfer (grooves, ribs and extended cooling surfaces, etc.), installation of draw stack with significant height for increasing the velocity of air propagation. The rate of heat removal by PEHRS, as well as heat removal in dry cooling towers influence the NPP operational performance, it can vary under the effects of different ambient factors, such as ambient air temperature, sun radiation, side wind, rain and snow (Yang et al. 2013; Fahmy and Nabih 2016; Li et al. 2018). The location of the NPPs plays an important role in the efficiency of the equipment. Construction of NPPs in such countries as Iran, Iraq, Bangladesh, Egypt, Saudi Arabia, etc. with high air temperatures (up to 55 °C in the summer) creates additional problems for designing modules of PEHRS heat exchangers and dry cooling towers. Development of advanced more compact and efficient cooling technologies suitable for operation at high ambient air temperatures constitutes an important task. It is demonstrated in the present paper that high intensity of heat transfer with a significant reduction of dimensions and weight of heat exchange modules can be achieved using water finely dispersed in air. Application of the air-water mist cooling method is achieved by the injection of micro-size water droplets in the air cooling the heat exchange surfaces. Use of air-water mist as the working medium for dissipating heat is preferable as compared with purely air-cooling. The main distinguishing features of cooling using air-water mist as compared with single-phase air cooling can be explained as follow: absorb large quantities of thermal energy in the process of evaporation, increase of heat capacity of the cooling mixture, and increase the turbulence in the boundary layers of the air flow (Bhatti and Savery 1975; Wataru et al. 1988; Wang and Dhanasekaran 2010). The heat transfer mechanisms can include the following three important effects: convection, conductive heat transfer and evaporation of water droplets. The last two effects are weakly dependent on the external factors and ensure active heat removal within wide range of ambient temperatures. This technology can be used in the projects of upgrades of existing and development of new PEHRS, as well as dry cooling towers of nuclear and thermal power plants operated in the conditions of the hot climate. The aim of the present study was to implement an experimental investigation of the effects of air-water mist droplets on the heat transfer intensification. Experiments were conducted using a copper calorimetric sphere as the model of the heated surface at various densities of heat flux, water concentrations and velocities of airflow following the methodology represented in (Hayashi et al. 1990, 1991).

According to Dalton’s law, the rate of evaporation from the surface of a body W in kg/(m^2.s) is directly proportional to the moisture deficit calculated using the temperature of the evaporating surface and inversely proportional to the atmospheric pressure (Huang et al. 2015):

W = A (X_{T, sph} – X_T,in)/P, (1)

where X_T, _sph is the elasticity of saturation taken using the temperature of the evaporating surface, GPa; X_T,_in is the elasticity of vapor in the ambient air, GPa; Р is the atmospheric pressure, GPa; А is the proportionality factor dependent on the air flow velocity.

It is clear from Dalton’s law that the higher is the difference X_T,_sph – X_T,_in the higher is the evaporation rate. If the evaporating surface is warmer than the air, then X_T,_sph exceeds X_T,_in. In such case, evaporation continues even when air is saturated with water vapor.

General layout of the test stand for investigation of processes of heat dissipation from the surface by air-water mist is presented in Fig. 1.

Figure 1. 

General layout of the experimental stand: 1 – plexiglass tube; 2 – support; 3 – copper sphere; 4 – blending chamber; 5, 15 – differential pressure gauge; 6 – fan; 7 – tank; 8 – ultrasonic mist generator; 9 – multimeter; 10, 17 – voltage controller; 11 – analogue output module; 12 – data collection module; 13 – computer; 14 – Pitot tube; 16 – air pump.

The test stand consists of the measuring section with the object under measurement (spherical element with embedded heater), air supply, mist generation, phase blending and data collection subsystems (temperature, water and air flowrates, pressure, etc.).

Measurement section is the transparent channel with 50 mm diameter, 2 mm wall thickness and total length of 940 mm. The measured object – a sphere with 34 mm diameter made of copper – is mounted inside the channel and is supported from the rear side with a fabric-reinforced laminate rod with 4 mm diameter.

Air supply subsystem incorporates electric vacuum air blowers (3000 rpm, 1000 W) with air flowrate controller, compact Pitot head equipped with digital micromanometer for measuring average airflow velocity.

Spherical element is heated using alternate current electrical heater installed in the stainless steel casing with 8-mm diameter and 31-mm length. Electrical heater power is controllable for achieving the required heat flux on the surface of the sphere using RNO-250-1000 autotransformer and is measured using APPA 109N digital multimeter with ± 0.06% precision.

Special high-temperature heat-conducting paste is used for reducing thermal resistance between the metal of the sphere and the electrical heater.

Schematic layout of the spherical element with heater and thermocouples is shown in Fig. 2. Five calibrated chromium-nickel thermocouples are applied for measuring temperature of the spherical surface as follows: two thermocouples are mounted inside the copper sphere (positions 1 and 2 in Fig. 2), one thermocouple is installed in the inlet flow zone and two thermocouples are positioned at the outlet of the flow after flowing around the sphere. Thermocouples 1 and 2 are caulked in the holes with 1-mm diameter drilled on the surface of the sphere and are thoroughly polished.

Figure 2. 

Layout of the spherical element: 1, 2 – thermocouples; 3 – thermal paste; 4 – electrical heater; 5 – fiber-reinforced laminate rod; 6 – support; 7 – copper sphere.

All thermocouples are connected to the data collection system consisting of the analogue signal input module of OWEN MV110-8A type, MSD200 data registration device and personal computer.

Air-water mist is generated as the result of injection of micro-size water droplets in the airflow inside the blending chamber. Ultrasonic mist generator with piezoelectric transducers and generator operated at 1.7-MHz frequency (Kudo et al. 2017) is applied as the mist generation subsystem. This type of mist generator was selected due to the low energy consumption and noiseless operation in contrast with other generators of mist and vapors. Dimensions of the droplets were calculated using Lang formula (Lang 1962) for ultrasonic spraying of the liquid and were found to be equal to 2.73 µm.

Mass flow rate of the sprayed water was determined according to the change of water mass in the tank of the mist generation subsystem per unit time. Mixture of finely dispersed water with controlled amount of air is formed in the blending chamber. Application of the process of air humidification under discussion here allows obtaining air-water mist with low mass humidity varying in the course of experiments within the range from 0.1 to 6%.

The precision of measurements of calorimeter temperature amounted to 4%. Uncertainties of measurements of air and water flowrates, heater power and geometrical characteristics ranged from two to 3%. Multiple repetitions of measurements at the preset regimes produced statistical dispersion of results within 5%.

The following dimensionless criteria were used in the processing of experimental results:

• Reynolds number Re = U× D/n, where U is the velocity of air flow in the channel, m/s; D is the internal diameter of the channel, m; n is the air kinematic viscosity, m ²/s;
• Nusselt number Nu = a× d/l, where a is the heat transfer coefficient, W/(m ²×K); d is the diameter of the sphere, m; l is the thermal conductivity of air, W/(m×K).

Uncertainties of calculation of Nusselt and Reynolds numbers amount to less than 10%, which allows reliable detection of the effect of heat transfer intensification.

Equation for the sum of convection Q_conv, conductive Q_cond. components of heat transfer and evaporation of water droplets Q_ev on the heated surface (Allais and Alvarez 2001, Allais et al. 2006) was used in the analysis of interaction between the heated surface and the flow of air-water mist:

Q = Q_conv. + Q_cond. + Q_ev., (2)

Q _conv. = aF_sph(T_av.sph – T_in), (3)

Q _cond. = M_dC_p,w(T_av.sph – T_in), (4)

Q _ev. = aF_wet. (X_T,sph – X_T,in)r/C_p,w, (5)

where a is the heat transfer coefficient; F_sph is the surface of the sphere; T_av.sph is the average temperature of the surface of the sphere; T_in is the inlet temperature of the mist; M_d is the water mass flowrate; C_p,w is the water heat capacity; F_wet is the area of the wetted surface of the sphere; X_T,sph is the concentration of moisture for the temperature of the sphere; X_T,in is the concentration of moisture at the temperature at the mist inlet in the channel.

It is assumed that part of water precipitates on the surface in the process of the mist flowing around the heated sphere. Part of water droplets evaporates, while some of them remain scattered in the main airflow. Experiments were conducted with pure dry air and with air-water mist within the range of water flow densities (j = 23.39, 46.79, 88.31, and 111.68 kg·m^–2·hr^–1). Range of Reynolds numbers is 2500 – 55000.

Test facility and methodology for investigation of heat transfer characteristics were developed for the case when the heat transfer surfaces of the model of PEHRS heat exchange element are cooled down by air-water mist. Investigation of heat transfer was performed within wide range of heat flux values, Reynolds numbers and concentrations of water droplets for the case of cooling of object with air-water mist. The following conclusions can be drawn based on the experimental results obtained.

Use of air-water mist is an efficient method for cooling heat exchange surfaces. Temperature of the surface decreases by 4, 17, 37 and 47% as compared with cooling by air flow for water flow densities j = 23.39, 46.79, 88.31 and 111.68 kg·m^–2·hr^–1, respectively.

Average Nusselt number significantly increases with increased Reynolds number for all values of heat flux and water flow densities. When finely dispersed water is used, the average Nusselt number is found to be 11, 19.7, 90.2 and 134% higher than for the case of cooling with air flow for water flow densities j = 23.39, 46.79, 88.31, 111.68 kg·m^–2·hr^–1, respectively. Average Nusselt number increases with increased heat flux for fixed values of water flow density and Reynolds numbers.

The value of the factor characterizing heat transfer intensification in the case of cooling with air-water mist flow is significantly higher (up to 300%) than for the case of air cooling and has area of optimal moisture concentration for each value of water flow velocity.

The present study was implemented under the financial support in pursuance with Resolution No. 211of the Government of the Russian Federation under Contract No. 02.A03.21.0006.