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For spaces with occupants coming in and out frequently, e.g., airport terminals and commercial buildings, the intermittent opening of the entrance door causes masses of cold outside air to invade the heating space in winter, and this leads to an increase in the heating energy demands. As impinging jet ventilation (IJV) has a potential for reducing high energy consumption compared to the use of mixing ventilation (MV), a series of experiments were conducted to investigate the temperature distribution and energy consumption of IJV used in a space with cold air invading intermittently and compared with those in a MV system. The results indicate that the door opening frequency only affects the amplitude of temperature fluctuation with time for a fixed door opening duration. The floor-to-ceiling temperature difference in the IJV is always less than 60% of that in the MV for different durations of door opening, proving that the former consumes less heating energy. Moreover, the increase of the inertia to buoyancy ratio on the supply air can improve the thermal environment and reduce energy consumption for both IJV and MV, with this improvement better in IJV than in MV, meaning that IJV is more energy efficient than MV when used for heating space with cold air invasion. [ABSTRACT FROM AUTHOR]

Abstract: Full-scale experiments were carried out to explore the effects of the initial temperature difference between the interior and exterior and the vent characteristics on the transient development of natural ventilation driven by thermal buoyancy. Air temperature and tracer gas concentration in a test chamber were measured and the experimental results were compared with the theoretical predictions. It was found that the initial temperature difference has a large influence on the thermal stratification, the concentration distribution of tracer gas, the flow rate and the removal rate of tracer gas. The time taken to reach steady-state ventilation is shorter for a larger vent if the initial indoor temperature is greater than or equal to the outdoor temperature. For a pre-cooled room, the time taken for the ventilation to transform the airflow direction is shorter for a larger vent. Increasing the vent area would yield a greater flow rate and thus improve the efficiency of gas removal. Experimental results also show that the vent shape has little impact on the flow rate. [Copyright &y& Elsevier]

A modified theoretical model is developed to examine the unsteady natural ventilation produced by a localized inner heat source at the floor of a room. The model is validated by using experimental results in the literature. The main difference between this model and previous models is that the buoyant layer is regarded as composed of a middle layer and a near-ceiling layer rather than being well-mixed. This model is therefore named the three-layer model and then used to investigate the unsteady process. Three characteristic times are defined to predict the time taken for the stratification interface to reach three particular positions during the transient process. When the dimensionless effective vent area is smaller, the steady-state depths of the near-ceiling layer and the middle layer are smaller and larger, respectively. The dimensionless buoyancies of the near-ceiling layer and the middle layer increase gradually over time while the plume buoyancy at the lower interface increases rapidly to its maximum and then decreases slowly. The temperature difference between the near-ceiling layer and the middle layer is small, but it is not negligible comparing to the temperature difference between the middle layer and the lower layer. The variations of the layer buoyancy and ventilation flow rate with time are both dependent on the source buoyancy flux, the floor area, the effective vent area and the enclosure height. [ABSTRACT FROM AUTHOR]

Inappropriate thermal stratification characteristics can degrade the ventilation performance of impinging jet ventilation (IJV). In the present study, an analysis method using dimensionless parameters is proposed for predicting the thermal stratification, and the Box-Behnken design method is employed to determine the simulation cases of influencing factors for thermal stratification. The thermal performance in IJV cooling room is investigated by computational fluid dynamics (ANSYS Fluent), and the effects of supply air parameters, inlet area (S *) and height (h *) on thermal stratification are discussed, respectively. A further supply air parameter is represented by the thermal length scale (L m), which is applied to evaluate the combined effect of inertial force and thermal buoyancy on supply airflow. The results show that the driving force on supply airflow changes from buoyancy dominated to inertial dominated with increasing L m , leading to the thermal stratification profiles changing from thermal stratification to uniform distribution. For small L m , the thermal stratification profiles are similar for different inlet areas, while the dimensionless air temperature decreases with increasing inlet area. However, the larger inlet area promotes a more uniform temperature distribution for large L m. In addition, the thermal stratification profiles are similar regardless of the inlet height for both small and large L m conditions, indicating that the dimensionless air temperature is a weak function of inlet height. Finally, multiple regression models related to L m , S * and h * are developed for predicting the thermal stratification, which can provide a reference basis for the determination of temperature distribution in IJV cooling rooms. ● Thermal stratification (TS) of impinging jet ventilation (IJV) is studied. ● L m is used to evaluate the combined effect of inertial and buoyancy forces on TS. ● Prediction models of TS are developed for IJV cooling rooms (θ ank , Δ θ oz and Δ θ fc). ● Effect of L m on Δ θ fc is less than that on θ ank and Δ θ oz. [ABSTRACT FROM AUTHOR]

Thermal stratification is the key to improving energy efficiency and indoor air quality for impinging jet ventilation (IJV) system in cooling mode. However, unclear thermal stratification characteristics of IJV system can degrade the ventilation performance of the IJV. In the present study, the thermal stratification characteristics of an IJV is investigated by the validated simulation model for different supply air parameters and internal heat sources in interior zone of office buildings. The critical value of supply air parameters for the formation of thermal stratification is determined by thermal length scale, which considers the combined effect of inertial force and thermal buoyancy on the supply airflow. In addition, the cooling load in an IJV cooling room caused by the heat source intensity can be calculated directly and the heat source distribution is negligible. The results also indicate that the thermal stratification height (H) only depends on supply air parameters, while the temperature difference between head and ankle level (Δ T) is affected by both supply air parameters and cooling load. The predictive models of H and Δ T are obtained based on the simulation results, which could provide the theoretical basis for optimizing the design scheme of IJV system in interior zone of office buildings. • Thermal stratification (TS) of impinging jet ventilation for cooling is studied. • Critical value of supply air parameter (L m) is determined for the formation of TS. • Thermal stratification height H is mainly affected by L m but not cooling load (q). • Temperature difference between head and ankle (Δ T) is affected by both L m and q. • Predictive models of H and Δ T are obtained for design of impinging jet ventilation. [ABSTRACT FROM AUTHOR]