CFD Analysis of Residential Kitchen Ventilation with Gas Stove

Proper ventilation of the kitchen is essential to reduce the occupant exposure to various pollutants generated during the cooking and the combustion of natural gas through the gas stove burners. Several parameters related to the design and operation can affect the performance of kitchen ventilation. Exhaust flow rate is one of the key parameters that determines the level of dilution and rate of removal of pollutants from the kitchen. Field measurements show that pollutant concentrations vary widely within the homes, and sometimes, exceed the acceptable levels. Pollutants can migrate from the kitchen to adjacent spaces such as the living room, bedroom, etc.

Airflow Patterns Matter!

Airflow patterns in the kitchen impact the flow behavior of pollutants. Understanding the nature of airflow patterns and the flow path of pollutants plays a critical role in determining the pollutant exposure levels of occupants. The dispersion of pollutants and their flow path can depend on several inter-related factors. These factors include exhaust flow rates, design of the exhaust hood, size and location of make-up air openings, location, and capacity of the burners used during cooking, cooking practices, and user behavior in operating the kitchen ventilation system. Physical testing of all these parameters under controlled conditions for a variety of conditions is time and labor-intensive, if not impossible. Computational Fluid Dynamics (CFD) analysis provides a feasible alternative for systematic evaluation of these parameters and gaining insights into the performance of kitchen ventilation.
With the help of CFD simulations, we investigated the impact of exhaust fan flow rates on the spread of Nitrogen Dioxide (NO₂) and its concentration levels in a typical kitchen. Unlike our previous study with the mass balance approach which predicted only a “volume-averaged” concentration, this CFD study predicts the spatial distribution of NO₂ concentration in the space.

Analysis of Exhaust Fan Flow Rates

Our CFD analyses indicate that the exhaust flow rate of 75 cfm or less is inadequate for the effective removal of pollutants from a kitchen space. Increasing the exhaust flow rate reduces the spread of pollutants and high-concentration zones. High exhaust flow rates also remove the pollutants quickly from the kitchen. Turning on the exhaust fans in the nearby bathroom can supplement the capacity of the hood exhaust fan.

Figure 1: Spread of Nitrogen Dioxide (NO₂) in the kitchen and the adjacent living room during and after the cooking. It shows a percent of the space volume with a NO₂ concentration of at least 100 ppb. It shows that the exhaust fan capacity of 75 cfm is inadequate to control the spread of NO₂. Higher exhaust flow rates remove the pollutants quickly after the cooking and the thermal plume rising from a person can form a “protective shield”.

Thermal Plume Can Protect!

These analyses further indicate that the thermal plume – hot air rising from a person and the buoyancy of hot gases released during the cooking can help reduce the NO₂ exposure. Such thermal stratification causes the pollutants to accumulate near the ceiling and migrate along the ceiling. Thus it moves the pollutants up and away from the breathing zone of occupants. Thermal plume from a person can form a “protective shield” which helps reduce pollutant exposure.

Figure 2: Distribution of temperature in the kitchen and the adjacent living room during and after the cooking. It shows thermal stratification with the hot air near the ceiling with a thermal plume from the person.

During the first few minutes of the cooking when the burner is ON, the occupant NO₂ exposure levels remain low, however, the exposure level starts increasing during the purge phase when the burner is turned off. Interestingly, this analysis indicates that the NO₂ exposure level remains significantly below the EPA acceptable level of 100 ppb-hr for all exhaust fan flow rates.

CFD is a valuable tool in analyzing and optimizing kitchen ventilation systems.