Model of Thermal Plume above Cooking Gas Stove for Designing Ventilation

A model of the thermal plume above a cooking gas stove using computational fluid dynamics (CFD) analysis was studied to predict the heat and vapor released during cooking. The combustion gas released from the burner installed in the gas stove was considered as air in which thermal energy was adjusted so that the thermal plume above the gas stove could be simulated. Therefore, the model could predict the thermal plume above the gas stove based on the capacity of the burner and pot size. For validating the simulated flow fields, the results of the velocity distributions above the gas stove calculated using CFD analysis models were compared with the results of the velocity distributions measured with particle image velocimetry (PIV). In conclusion, the analysis results were in good agreement with the measurement results. However, the velocity in the vertical direction calculated using CFD above the center of the burner was higher than the velocity measured using PIV along the axis from the center of the burner.


Introduction
Computational fluid dynamics (CFD) analysis is often used to design ventilation in commercial kitchens. The heat and vapor released during cooking negatively affect the environment in the kitchen. Therefore, appropriate ventilation must be designed to balance environmental degradation with efficient utilization of energy. For designing appropriate ventilation, CFD analysis can be used to calculate the capture efficiency of the hood for different types of kitchen appliances and different disturbance conditions. Several CFD models for calculating the capture efficiency of the exhaust gas released from the gas stove have been studied [1][2][3]. In these models, the measured data of the velocity distribution corresponding to the size of each burner and pot is needed to set the boundary conditions. Omori et al. proposed the exhaust gas plume model [4]. The combustion gas released from the burner installed in the gas stove was considered as air in which thermal energy was adjusted so that the thermal plume above the gas stove could be simulated. Therefore, the model could predict the thermal plume above the gas stove without the measured data of velocity distributions. However, it is not verified whether the velocity distribution corresponding to the pot shape can be simulated.
Hence, in this study, a model of the thermal plume above a cooking gas stove based on the Omori's model [4] was developed for different pot diameters. For validating the simulated flow fields, velocity distributions above a commercial cooking gas stove were measured using particle image velocimetry (PIV). A gas stove equipped with a 14.5-kW burner was placed in a test room composed of a ventilation system that guaranteed undisturbed flow from the system. The cooking mode was evaluated with boiling water in the pot. The results of velocity distributions above the gas stove calculated using CFD analysis models were compared with the results of velocity distributions measured using PIV. In conclusion, the analysis results showed good agreement with the measurement results.

Model of combustion gas
We considered the gas released from the gas stove as air in which thermal energy was adjusted to simulate the thermal plume above the gas stove. Figure 3 shows the model used for representing the combustion gas. The natural gas passing through the burner burns transmits heat to the pan and gas stove and flows over the pan. (1)

Qex = Qin -Qeff -Qpan -Qtab.
(2) Heat transfer through the pot, radiant heat transfer energy of the pot, and heat transfer through the gas stove can be described as follows: Table 1 presents the analysis cases. Each parameter was determined in correspondence with the gas stove used for measurements in the PIV tests and heating value of natural gas in Japan. The heating thermal efficiency of the gas stove was measured using the pot with diameters of 330 mm, 390 mm, and 440 mm in advance. Table 2 lists the boundary conditions. We set a boundary condition of floor as velocity inlet to stabilize the calculation. The boundary condition for the burner was set based on the model of the combustion gas.    Table 3 gives the analysis conditions. The mesh size was 10 mm, and the number of cells was 560,000.  The laser frame straddle time was adjusted to ensure that the tracer particle moved no more than 5 px during this time; consequently, it was set to 1300 µs for the vertical cross section and 500 µs for the horizontal cross section. The laser-pulse frequency was 10 Hz, which was the maximum value permitted by the device. The statistical data comprised 2000 images from the two-dimensional PIV test and 4000 images from the stereo PIV test. The result was arranged into a time-averaged flow field. The airflow was supplied from the displacement ventilation system (FLOORMASTER; Takasago Thermal Engineering Co., Ltd., Tokyo, Japan), which guaranteed an undisturbed convection flow from the ventilation system. Olive oil mist was selected as the tracer particle. Olive oil mist particles were released from three oil mist generators in the cooking mode.

Measurement methods for thermal distributions
The difference between the plume temperature at a height of 1000 mm from the top of the gas stove and room temperature was measured with hot wire anemometers. The measurement was not performed simultaneously with PIV because the anemometers would disturb the fluid velocity. The flow rate of the input gas was measured with a mass flow meter (CMG500; Azbil, Fujisawa).

Comparison of the CFD results and experimental results
(a) Two-dimensional PIV test for vertical cross section (b) Stereo PIV test for horizontal cross section  The calculated results were in good agreement with the measurement results. However, the velocity in the vertical direction calculated by CFD above the center of the burner was higher than the velocity measured by PIV along the axis from the center of the burner. Kiyosuke et al. suggested that standard k- models predict the insufficient diffusion in the buoyant plume, so the simulated capture efficiency of exhaust gas released from the gas stove tends to be overestimated [5]. Therefore, investigating the turbulence model for predicting the thermal plume is necessary in future works, to predict the capture efficiency accurately.    height affects the capture efficiency. The profile of the temperature rise calculated using CFD agreed with the profile obtained experimentally regarding the effect of the diameter of the pot on the thermal plume. However, the velocity in the vertical direction calculated using CFD above the center of the burner was also higher than the velocity measured using PIV along the axis from the center of the burner.

Conclusions
A model of the thermal plume above the cooking gas stove using CFD analysis was studied based on the present study to predict the heat and vapor released during cooking.
The analysis results calculated were in good agreement with the measurement results obtained via the PIV test. However, the velocity along the vertical direction calculated using CFD above the center of the burner was higher than the velocity measured using PIV along the axis from the center of the burner. Therefore, investigating the turbulence model for predicting the thermal plume is necessary in future works to predict the capture efficiency accurately.