19 university students were asked about their thermal comfort while attending ordinary lessons in a displacement ventilated test room of typical classroom size. Two different ceiling heights were tested. Both the general temperature level and the strength of the vertical temperature stratification in the room increased continuously during the lessons due to the presence of the students, however slower with the higher ceiling.
Most indoor airflows are mixed convection. In order to simulate mixed convection accurately and efficiently, this paper uses a two-layer turbulence model. The two-layer model combines a one-equation model for near wall flow together with the standard k-E model for outer-wall flow. The model has been used to predict the mixed convection by displacement ventilation in an office. The computed results agree well with the corresponding airflow pattern and the distributions of air temperature , air velocity, air velocity fluctuation, and tracer-gas concentration.
Temperature efficiency is an important index to estimate the ventilation effectiveness. Usually ,the temperature efficiency is determined through field or model tests such as gas-tracing technology. The heat source structure(location, size, heat emission, etc) has a strong effect on the temperature efficiency. The heat sources present themselves or may be arranged in three basic models:(A)heat sources uniformly distributed in the space; (B) heat sources uniformly distributed on the floor; (C)concentrated heat sources at the bottom of a room.
An important element in the natural ventilation design procedure is the flow-pressure characteristics of a window with a given opening area. The flow in the room is another important element that is often ignored in the design phase due to lack of relevant information on the air movement. This paper shows the outcome of experiments with the room air distribution. The results show that the velocity distribution in the occupied zone can be described by a semi empirical model.
This paper investigates the relationship between the neutral height for air distribution and the ventilation load in a room with displacement ventilation. An environmental chamber equipped with a displacement ventilation system has been used to carry out the neutral height measurements with the presence of a heated mannequin and other heat sources in the chamber. The total room load used was varied from 104 W to 502 W, i.e., corresponding to a ventilation load from 10 W/m2 to 60 W/m2. The prediction of the neutral height was based on plume theory.
In displacement ventilation the airflow pattern in a room is mainly guided by the convection flows from the heat sources present in the room. This implies that the air in the breathing zone mostly comes from the lower parts of the room, where the air often is less polluted by pollutants originating from persons or electrical appliances present in the room.
A study is being conducted to assess the performance of displacement ventilation in high-ceiling areas such as commercial and industrial manufacturing facilities. These areas, which can range from 5 to 20 meters in height, often feature high internal heat loads and contaminants associated with heat sources. Very little performance data exists for displacement ventilation installations in high-ceiling areas, particularly any which account for the influence of wall temperature.
The paper presents a comparison between the results of experimental tests airflow pattern forming in a room with displacement ventilation and numerical calculation. The heat source in the room was a heating plate. Quasi-laminar diffusers supplied the air with the ventilation change rate from l to 7 h-1. Temperature and velocity distributions in the plume and in its surroundings as well as the tracer gas concentrations in the background were measured. The airflow in the room was also predicted by means of CFD, using the standard k-E turbulence model and standard log-law wall-functions.
Two-dimensional computational simulations are performed to examine the effect of vertical location of a convective heat source on thermal displacement ventilation systems. In this study, a heat source is modeled with seven different heights from the floor (0.5m, 0.75m, 1.0m, 1.25m, 1.5m, 1.75m, 2.0m) in a displacement ventilation environment. The flow and temperature fields in thermal displacement ventilation systems vary depending on the location of the heat source. As a heat source rises, the convective heat gain from the heat source to an occupied zone becomes less significant.
In this study the instantaneous temperatures and velocities close to a diffuser for displacement ventilation have been recorded by using whole-field measuring techniques. The air temperatures were measured indirectly by the use of a low thermal mass screen in conjunction with infrared thermography. The measuring screen was mounted parallel to the airflow, acting as a target screen. By using the thermal images the size of the near zone was also calculated. To determine air movements a whole field method called particle streak velocimetry (PSV) was used.
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