Experimental and numerical investigations of heat transfer and thermal efficiency of an infrared gas stove

Transcription

1 IOP Conference Series: Materials Science and Engineering PAPER OPEN ACCESS Experimental and numerical investigations of heat transfer and thermal efficiency of an infrared gas stove To cite this article: A. Charoenlerdchanya et al 18 IOP Conf. Ser.: Mater. Sci. Eng View the article online for updates and enhancements. This content was downloaded from IP address on 28/03/19 at 12:52

2 Experimental and numerical investigations of heat transfer and thermal efficiency of an infrared gas stove A. Charoenlerdchanya 1, P. Rattanadecho 2, and P. Keangin 1,* 1 Department of Mechanical Engineering, Faculty of Engineering, Mahidol University, 25/25 Phutthamonthon 4 Road, Salaya, Nakhon Pathom 73170, Thailand. 2 Department of Mechanical Engineering, Faculty of Engineering, Thammasat University (Rangsit Campus), 99 mu 18, Paholyothin Road, Klong Nueng, Klong, Pathumthani 121, Thailand. * Corresponding Author: pornthip.kea@mahidol.ac.th Abstract. An infrared gas stove is a low-pressure gas stove type and it has higher thermal efficiency than the other domestic cooking stoves. This study considers the computationally determine water and air temperature distributions, water and air velocity distributions and thermal efficiency of the infrared gas stove. The goal of this work is to investigate the effect of various pot diameters i.e. 2 mm, 2 mm and 260 mm on the water and air temperature distributions, water and air velocity distributions and thermal efficiency of the infrared gas stove. The time-dependent heat transfer equation involving diffusion and convection coupled with the time-dependent fluid dynamic equation is implemented and is solved by using the finite element method (FEM). The computer simulation study is validated with an experimental study, which is use standard experiment by LPG test for low-pressure gas stove in households (TIS No ). The findings revealed that the water and air temperature distributions increase with greater heating time, which varies with the three different pot diameters (2 mm, 2 mm and 260 mm). Similarly, the greater heating time, the water and air velocity distributions increase that vary by pot diameters (2, 2 and 260 mm). The maximum water temperature in the case of pot diameter of 2 mm is higher than the maximum water velocity in the case of pot diameters of 2 mm and 260 mm, respectively. However, the maximum air temperature in the case of pot diameter of 260 mm is higher than the maximum water velocity in the case of pot diameters of 2 mm and 2 mm, respectively. The obtained results may provide a basis for improving the energy efficiency of infrared gas stoves and other equipment, including helping to reduce energy consumption. 1. Introduction In the present, the domestic gas stoves are separated with three types, which are low-pressure stove, high-pressure stove and infrared gas stove. Process method of infrared gas stove is infrared radiation (IR). Infrared radiation is classified with wavelength in 3 types, namely near-infrared radiation: NIR ( µm), middle-infrared radiation: MIR ( µm) and far-infrared radiation: FIR ( µm) [1]. Most of burner type of infrared gas stove that use in Thailand is ceramic burner. Because of burner type ceramic match with domestic cooking more than the other types. From table 1, wavelength and maximum burner temperature of ceramic burner is appropriate for using when compared with the other types. Consider magnetic wavelength reverse variation with temperature, the burner in far-infrared Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1

3 radiation will make food absorb the radiation as well. In the other side, the burner in far-infrared radiation will use more time to make food ripe. In addition, the burner in near-infrared radiation will use less time to make food ripe but maybe inside of food is not ripe. Therefore, the ceramic burner is gas stove with infrared radiation medium that is suitable. Table 1. Comparison of the properties of burner [1]. Parameters Types of burner Catalytic burner Ceramic burner Metal fiber burner Porous burner Wavelength category Long-wave Medium-wave Medium-wave Short-wave Peak wavelength 3.5 µm 2.4 µm 2.2 µm 1.7 µm Maximum burner 850 K 1100 K 10 K 10 K temperature attained Maximum thermal load 30 kwm -2 1 kwm -2 0 kwm kwm -2 Infrared gas stove is compared with the other domestic gas stove and the result is infrared gas stove saves energy and decreases gas consumption more than the other types. From data of entrepreneur of infrared gas stove found that commercial quantity is 430,000 stoves per year. Comparing to the commercial quantity of infrared gas stove 2 years before, the quantity increases by 14 % [1]. This report shows that the people are increasingly interested in infrared gas stove and realize the way to save energy and decrease gas consumption. According to previous researches, there is little research about infrared gas stove. Many previous researches studied about the thermal efficiency and gas emission of burner by experiment such as comparison between the types of burner and percentage of thermal efficiency that porous radiant recirculated burner (PRRB) has thermal efficiency more than standard burner (SD) about 10 % [2]. Porous radiant recirculated burner (PRRB) was compared with conventional domestic burner (CB) in air temperature 300 C condition; it was found that the thermal efficiency of PRRB increases about 12 % [3]. In swirling central flame ring condition, the maximum thermal efficiency of PRRB is about 60 % [3]. Comparing porous radiant burner (PRB) with LPG conventional domestic stove found that PRB has higher thermal efficiency than LPG conventional domestic stove and CO and NO x emissions of PRB were less than LPG conventional domestic stove [4, 5]. Several researchers studied the parameters that effect on the thermal efficiency. Thermal efficiency of conventional open flame gas cooker with swirling central flame burner was higher than conventional radial flow burner about 15 % [6]. Different design of burner heads were compared and found that flat face burner (Brass) has highest thermal efficiency [7]. From three-dimensional computational fluid dynamic (CFD), thermal efficiency of conventional domestic burner with modified design increased about 2.5 % [8]. There were some numerical studies of the domestic burner model. One-dimensional numerical model of reticulated ceramic radiant burner can predict burner flame structure by compared temperature with thermocouples measurements [9]. In this paper aim to study about the effect of the diameters of pot on the water and air temperature distributions, water and air velocity distributions and thermal efficiency of the infrared gas stove during heating by using infrared gas stove. In order to verify the accuracy of the present model, the computer simulation study is validated with an experimental study. The test method is referenced from the lowpressure gas stove in households with LPG test (TIS No ) and the Development of energy efficiency standards infrared burner project. An axially symmetric model of pot and infrared gas stove are considered in this study, which minimized the computation time while maintaining good resolution and represent the full three-dimensional results. The governing equations as well as initial and boundary conditions are solved by using the axisymmetric finite element method (FEM). The range of temperature and velocity are separated by the different colours that are convenient to analyse the results. The results will be the basis for the study and analysis about the effects of various parameters on temperature 2

4 distribution, velocity distribution and thermal efficiency during heating by using infrared gas stove. The complete mathematical model is useful for the development of heating process by infrared gas stove technologies and guide the development of effective infrared gas stove to lead the way in energy saving way. 2. For experimental, the test method is referenced from the low-pressure gas stove in households with LPG test (TIS No ) and the Development of energy efficiency standards infrared burner project. Figure 1 shows an infrared gas stove used in the experiment. The infrared gas stove is composed of ceramic plate that it emits heat to the pot. Experimental methods to determine the thermal efficiency of the infrared gas stove in this study are in the following steps. Starting with check and prepare experimental equipment, set up stove and connect the stove with gas gauge, pressure equipment, gas temperature gauge and valve gas. Then adjust the gas valve to the maximum gas inject position at gas pressure of 280 millimeter water (mm H 2O). Warm the burner for 5 minutes to remove stains dirt and oil. Turn off the stove and place the pot on the center of stove then put the thermometer to measure temperature. The initial water temperature should be set up at 25 ± 2 C. In addition, the testing room is controlled room temperature at 25 ± 2 C. Before the test, the temperature of gas and water temperature are recorded. The position for measuring the temperature of the center of the water at the height of 5 mm from the bottom of the pot is considered. Follow this procedure until the temperature of water increase from initial for 50 C. Finally, turn off the stove; record the last temperature of water and quantity of gas. An experiment is repeated 5 times. The data of the test will be used to calculate the thermal efficiency of infrared gas stoves. Before do the test again, pot cleaner clearly to minimize heat loss. Figure 2 shows the experimental setup. The temperature measured in experimental is recorded by using digital thermometer during heating. The dimension of pot and infrared gas stove and conditions in this experiment setup is used for the simulation. Figure 1. Infrared gas stove used in the experiment. Figure 2. The experimental setup. 3. Analysis of heat transfer in water during heating by using infrared gas stove is presented. The system of governing equations as well as initial and boundary conditions are solved numerically using the FEM via COMSOL TM Multiphysics Physical model In order to verify the accuracy of the present model, the simulation results are validated against the experimental results with the same conditions. The mathematic model used in computer simulation is divided into two parts, namely a container (pot) and infrared gas stove. The pot made of aluminum material. Because of the pot and burner are cylinder shape and the pot is located on the center of burner, 3

5 an axially symmetric model are considered in this study for save the computation time. As shown in figure 3, the axially symmetric model of pot and infrared gas stove for heat transfer analysis of water heating by using infrared gas stove is proposed. Heat absorbed by the water and converted into internal heat generation, which causes its temperature of water to rise. The convergence test is carried out to identify the suitable numbers of element required. The number of elements where solution is independent of mesh density is found to be 431,110 elements. It is reasonable to confirm that, at this number of element, the accuracy of the simulation results is independent from the number of elements through the calculation process Equations for heat transfer analysis The heat transfer model is developed to predict the temperature distribution in the water and air. The mathematical model in this study is shown in figure 3. The assumptions have been offered for computer analysis is as follows: lack of the combustion of gas fuel, unconsidered a container base and no phase change and no chemical reaction in the water. Also, thermal properties of water, aluminum pot and air are constant. The initial condition of water is defined as = 25 C at t = 0 s in order to comply with the experiment. The governing equation describing the heat transfer phenomenon is given in equation (1): T 0 T c cu T ( k T) 0 t (1) where = 1000 kg/m 3 is the density, c = MJ/kg.K is the specific heat capacity, T is the temperature ( C), t is the time (s), u is the velocity (m/s) and k = W/m K is the thermal conductivity. Outflow nˆ ( k T 0) Cover Air Axial Symmetry Wall u 0 Thermal Insulation nˆ ( k T 0) Open Boundary Wall u 0 Pot Water Burner Inlet Un 0 Heat Continuity Wall u 0 Thermal Insulation nˆ ( k T 0) Temperature T t C Thermal Insulation nˆ ( k T 0) Figure 3. Physical model for this study. Figure 4. Boundary condition for numerical analysis. 4

6 3.3. Equations for fluid flow analysis Corresponding to heat transfer analysis, fluid flow analysis inside the water and air is assumed in axially symmetric model. Fluid flow analysis is formulated to describe the water and air velocity distributions. Using standard symbols, the governing equations describing the fluid flow are given as follows: Continuity equation: t u 0 (2) Navier-Stokes equation: u T 2 ( u ) u pl u u u l t 3 (3) where is the density (kg/m 3 ), t is the time (s), u is the velocity (m/s), the viscosity (Ns/m 2 ) and l is the identity matrix tensor. p is the pressure (Pa), is The equation for calculate the thermal efficiency is given in equation (4) [1]: m c ( T T1) Tg % V Q 298 P P P s m sat (4) where is the thermal efficiency (-), m is the mass (kg), c = MJ/kg.K is the specific heat capacity, T 1= 25 ± 2 C is the initial temperature of water, T 2 = 75 ± 2 C is the final temperature of water, V is the amount of gas used test (m 3 ), Q = MJ/m 3 is the lower heating value, temperature of the gas during the test ( C), P s is the atmospheric pressure during test (kpa), T g P m is the is the pressure of the gas during the test (kpa), P sat is the pressure of saturated stream at a temperature of gas during test (kpa). The boundary conditions for heat transfer analysis are shown in figure Results In this section, the experiment of water temperature distribution results and the results of the numerical are focused on the water and air temperature distributions, water and air velocity distributions and thermal efficiency during heating The comparison of water temperature of simulation results with the experiment results The accuracy of numerical model is verified by the validation against the experimental under the same geometric model and same conditions. The comparison of water temperature of simulation results with the experiment results, which varies with the three different pot diameters at 2 mm, 2 mm and 260 mm during heating by using infrared gas stove as presented in figure 5. It can be seen that the water temperature of simulation results are in good agreement with the water temperature of experiment results. There are , and average percentage errors in case of using diameters of pot at 2 mm, 2 mm and 260 mm, respectively. Certain amount of mismatch between the simulation results and the experiment results is caused by the numerical scheme. Figure 6 also indicates that an increase in the heating times results in an increase temperature. This favorable comparison lends 5

7 confidence in the accuracy of the present numerical model and ensures that the numerical model can accurately represent the phenomenon of heat transfer between heating the water using infrared gas stove Water temperature ( C) Water temperature ( C) Water temperature ( C) % Average Error = % % Average Error = 5.454% % Average Error = 6.411% (a) (b) (c) Figure 5. The comparison of water temperature of simulation results with the experiment results that vary by diameters of pot using (a) Pot diameter 2 mm, (b) Pot diameter 2 mm and (c) Pot diameter 260 mm The comparison of air temperature of simulation results with the experiment results As same as the conditions from the comparison of water temperature of simulation results with the experiment results, the comparison of air temperature of simulation results with the experiment results are considered. Because of the surrounding temperature will show the heat loss to the air. The results of the validation test cases are illustrated in figure 6 and clearly show good agreement of air temperature between the simulation results and that of experimental results. For all pot diameters, the results of these simulations are consistent with experimental results. There are 1.894, and average percentage errors in case of using diameters of pot at 2 mm, 2 mm and 260 mm, respectively. It is found that an increase in the heating times results in an increase air temperature. In addition, it can be seen that the air temperature increase corresponds to higher diameters of pot in the same time Air temperature ( C) Air temperature ( C) Air temperature ( C) % Average Error = 1.894% % Average Error = 2.483% % Average Error = 2.021% (a) (b) (c) Figure 6. The comparison of air temperature of simulation results with the experiment results that vary by diameters of pot using (a) Pot diameter 2 mm, (b) Pot diameter 2 mm and (c) Pot diameter 260 mm The effects of diameter of pot Figures 7(a) (c) show the effects of pot diameter on the patterns of the water temperature simulated results at heating times of 780 s during water heating by using infrared gas stove. The diameter of pots of 2 mm, 2 mm and 260 mm are considered. It is evident that the water temperature in the case of diameters of pot at 260 mm has the highest temperature because the bottom of the pot is larger which heat is received from the burner than other cases. Heat energy is passed to a container and is delivered to water and thereafter the absorbed energy is converted to thermal energy, which increases the water temperature. Figures 8 display the simulation results of water velocity distribution at different pot diameters (2 mm, 2 mm and 260 mm). The distributions of the water velocity simulated results at 6

8 Water velocity (m/s) heating times of 780 s during water heating by using infrared gas stove are investigated. It can be found that the water velocity in the case of diameters of pot at 2 mm is higher than the water velocity in the case of diameters of pot at 2 mm and 260 mm, respectively. Because of the velocity of the big area will less than the velocity of the small area. The patterns of the air velocity as shown in figure 9(a) (c). The flowing air with higher velocity acts as a heat sink and results in dissipates the thermal heat to the surrounding air. From the simulation results are presented in figure 8 and figure 9, the water velocity is very small as compared to the conventional problems. However, the main idea behind this research is to propose the completed model in order to completely explain the actual heat transfer during water heating (a) (b) (c) Figure 7. The water temperature patterns of simulation results that vary by diameters of pot using (a) Pot diameter 2 mm, (b) Pot diameter 2 mm and (c) Pot diameter 260 mm. x Pot diameter 2 mm Pot diameter 2 mm Pot diameter 260 mm Figure 8. Comparison of the water velocity of simulation results at various diameters of pot at heating times of 780 s. 7

9 Thermal efficiency (%) (a) (b) (c) Figure 9. The air velocity patterns of simulation results that vary by diameters of pot using (a) Pot diameter 2 mm, (b) Pot diameter 2 mm and (c) Pot diameter 260 mm The thermal efficiency The thermal efficiency of simulation results that use the quantity of gas consumed by experiment as showed in figure 10. It is found that the thermal efficiency of pot diameter 260 mm is higher than thermal efficiency of pot diameter 2 mm and 2 mm, respectively because the lager pot size has a large temperature gradient produced by the heat energy, causing a strong impact of natural convection that gives a buffer characteristic to the water temperature during water heating process Thermal efficiency of simulation results Pot diameter (mm) Figure 10. The thermal efficiency of simulation results at various diameters of pot. 5. Conclusion This research presents the experimental and numerical analysis of water heating using infrared gas stove. The water and air temperature distributions, water and air velocity distributions that vary by three different pot diameters (2 mm, 2 mm and 260 mm) are considered. The conclusions of this study can be summarized as follows: 1) The results from the simulation are in agreement with the experimental data. 2) The water temperature distribution increase with greater heating time that vary by three different pot diameters (2 mm, 2 mm and 260 mm). The pot with larger area has higher quantity of water, so it makes more time to increase temperature to the final temperature (75 ± 2 C). 3) The air temperature distribution increase with greater heating time that vary by three different pot diameters (2 mm, 2 mm and 260 mm). The air temperature measured point of every pot diameters is at the same position, so the larger pot diameter was increase the air temperature more than the smaller 8

10 pot diameter. Because of the smaller pot has long destination between the air temperatures measured point and pot. 4) The water velocity distribution decreases with greater pot diameters that vary by three different pot diameters (2 mm, 2 mm and 260 mm). The reason of velocity trend is about the size of pot diameter and quantity of water, so the water velocity of the pot diameter 2 mm which is less area is more than the water velocity of 2 mm and 260 mm in the same time. 5) The air velocity distribution decrease with greater pot diameters that vary by three different pot diameters (2 mm, 2 mm and 260 mm) corresponds to water velocity distribution. The convective heat transfer characteristic has a strong effect on the air velocity distribution. 6) The average percentages thermal efficiency of three different pot diameters of 2 mm, 2 mm and 260 mm are , and , respectively. From the equation (4), the different water temperature between max and min values is effect to thermal efficiency, so the pot diameter 260 mm which has highest temperature at the last time has highest thermal efficiency. Acknowledgements The authors gratefully acknowledge Mahidol University for supporting this research. References [1] Department of Mechanical Engineering, Faculty of Engineering, Mahidol University, Final Report, Development of energy efficiency standards infrared burner project [2] Jugjai S, et al RERIC Int. Energy J. 18(2) pp [3] Jugjai S, et al. 02 Experimental Thermal and Fluid Science 25 pp [4] Pantangi V K, et al. 11 Energy 36 pp [5] Muthukumar P and Shyamkumar P I 11 Fuel 112 pp [6] Jugjai S, et al. 01 Int. J. of Energy Research pp [7] Khan M Y and Saxena A 13 Int. J. of Engineering Research & Technology 2 pp [8] Boggavarapu P, et al. 13 Fuel pp [9] Rumminger M and Dibble R 1996 Combustion/The Combustion Institute pp