Simulation by solutal convection of a thermal plume in a confined stratified environment: application to displacement ventilation

This paper describes the experimental study, by solutal simulation, of a thermal plume in a confined stratified environment, a situation encountered in displacement ventilation systems. The criteria enabling similarity to be established between the thermal plume in air and the solutal plume in a hydraulic model are discussed. Density stratification is detected by a planar laser-induced fluorescence (PLIF) technique. Criteria for defining the interface height and thickness are determined After validation of these criteria in the fully developed region of the plume in a confined stratified environment, a formulation of the stratification height in the region close to the source has been established.     

A study on a ventilation stack integrated with a light pipe

A theoretical and experimental study has been conducted on the performance of a vertical light pipe that also functions as an air flow stack for night ventilation. The rectangular light pipe of height 3 m and cross-section area 0.0625 m² surrounded by an air duct of total cross-section area 0.23 m² is situated above a room of height 3.8 m and floor area 9 m². Heat transfer from the hot water in the wraparound hot water jacket to the air in the duct is assisted by stainless steel fins. The ventilation of the room, due partly to the buoyancy of the air in the duct and partly to the wind effect, amounted to nearly 10 air changes per hour which is sufficient for passive cooling during cooler night periods. The light pipe has specular reflecting walls. It was found that the transmission of daylight through the light pipe in the middle of a partly cloudy day was sufficient for illuminating the room to general illumination level.     

Numerical–Analytical Assessment of Fire and Ventilation Interaction in Longitudinally Ventilated Tunnels Using Jet Fans

Longitudinal ventilation system is a frequent method of tunnel ventilation which is commonly implemented by use of jet fans. In most studies on longitudinal ventilation application, the effect of fire on its upstream is neglected and a constant-velocity flow is considered in the tunnel entrance. In this paper, fire consequences on ventilation system performance are investigated. It is noticed that airflow induced in tunnel declines as the fire intensifies. High reductions up to about 50% are observed and are attributed to two main phenomena; elevated downstream resistance and fire plume blockage. These effects are combined to form a parameter named fire pressure loss (FPL). An analytical model is also developed that gives acceptable predictions of FPL. Results illustrate that FPL increases with time due to gradual heatup of tunnel walls. Moreover, examination of jet fans operation shows that their efficiency declines when they get very close to fire upstream as a result of jet tilting and smoke stratification disruption. For fire downstream jet fans, the performance is deteriorated to an extent affected by installation height. Also, verification of flow patterns with experimental data is presented throughout the paper.     

高大空间粉尘污染的节能治理

根据JBJ10－96国家行业标准规定，具有变动尘源的厂房要采用全面通风进行通风除尘。但是，对于有变动尘源的高大厂房而言，采用全面通风不但运行费用高、耗能大，而且不能满足环保节能的要求。本文采用在厂房中部利用射流气流进行隔断，对厂房内的污染物分层分区处理，减少除尘系统能耗。     

Buoyancy-driven ventilation of hydrogen from buildings: Laboratory test and model validation

Hydrogen gas leaking from a hydrogen-powered vehicle in a residential garage may form a flammable mixture with air. Passive, buoyancy-driven ventilation is one approach to limiting the concentration to a safe level. We explored the relationship between leak rate, ventilation design, and hydrogen concentration through laboratory testing, an algebraic analysis, and CFD modeling. We used helium to test slow, steady, low-velocity leaks in a full-scale test room under nearly isothermal, steady conditions, and we report the results in sufficient detail that other modelers can use them. The results show the importance and variability of stratification. Our algebraic and CFD models agree very well with the experimental results. We describe our CFD approach in sufficient detail for use by others. We tested under nearly isothermal conditions, but also discuss indoor–outdoor temperature difference as an important risk factor. Information about realistic leakage scenarios is needed to apply these results as safety recommendations.     

Experimental,CFD simulation and parametric studies on modified solar chimney for building ventilation

The solar chimney is a passive solar system which can be used for enhance the natural ventilation and space conditioning of a building. A solar chimney design is modified and installed at CBRI Roorkee (29.87° N 77.88° E), India. A Computational fluid dynamics (CFD) software is used for prediction of velocity and temperature in Modified solar chimney (MSC) and evaluating the Air Change per hour (ACH), which is validated through experimental and theoretical counterpart and found a good agreement between them. From the result of thermal performance analysis, it is found that MSC generates 2.39–7.13 ACH in experimental room in month of May 2013, when outdoor solar radiation was in the range of 250–612 W/m². Due to this ACH, the room temperature is dropped by 2–4°C as compared to reference room temperature. The parametric study shows that the optimum glass tilt angle estimated by 5 degree for highest performance consideration of MSC. The air gap is optimised by 60 mm and air gap to inlet opening height ratio is optimised by 0.2.     

Non-adiabatic boundaries and thermal stratification in a confined volume

Heating experiments have been performed during 1.5–18 days in a confined room of volume about 300 m³ in an underground quarry near Paris (France). During heating, a thermal stratification of the atmosphere is observed. After a time depending on the presence or not of ventilation, this stratification reaches a stationary state during which the temperature difference between the rock and the atmosphere is constant. During this stationary phase, both temperatures increase linearly with time. We propose here a new model of filling box with radiative heat exchanges and cooling by direct contact at the boundaries, which accounts for the observed vertical profiles of temperature in the atmosphere, and for the temperature variation with time. Such contributions of heat transfer at the boundaries are important in situations such as fire in confined cavities, presence of visitors in a painted cave, or in the study of building ventilation.     

Experimental investigations on solar chimney for room ventilation

Experimental investigations on a small size solar chimney show that the rate of ventilation increases with increase of the ratio between height of absorber and gap between glass and absorber. This finding is in agreement with results of the steady-state mathematical model developed for analysis of such systems. Nine different combination of absorber height and air gap have been investigated on the experimental set-up. Highest rate of ventilation induced with the help of solar energy was found to be 5.6 air change per hour in a room of 27 m³, at solar radiation 700 W/m² on vertical surface with the stack height-air gap ratio of 2.83 for a 1 m high chimney.     

Onset of transition to turbulence in natural convection with gas along a vertical isotherm plane

It is generally accepted that the onset of transition to the turbulence in free convection along a vertical isotherm plane with gas occurs at a given Grashof number. A more subtle reality became apparent thirty years ago in the course of experiments carried out with either small installations and pressurized gas or under atmospheric pressure with an experimental installation of great height. To elucidate the problem, a re-examination of data set and a new analysis have been made. This has led to a physical interpretation of what was observed : the origin of the instability which is triggered off in this boundary layer lies in interaction between the zone beyond this boundary layer where there is a vertical stratification of temperature and the boundary layer near the plane. This is based on the comparison between characteristic times calculated for the two zones in question.     

An experimental study of the generation and characteristics of a two-layer thermally stable environment

An experimental system has been developed to generate a thermally stratified, two-layer, environment in an enclosure, with air as the fluid. Such a stably stratified circumstance, in which an essentially isothermal heated layer overlies a relatively cooler isothermal layer, frequently arises in several practical problems such as those related to heat rejection, heat transfer from isolated thermal sources in confined regions, and enclosure fires. A detailed study of the thermal field is carried out to determine the basic characteristics of the stratified environment and the dependence of the physical parameters such as interface location and temperature level on the governing variables. The location of the interface, between the two layers can be controlled by varying the temperature and the velocity of the heated fluid discharged at the top of the enclosure for stratifying the enclosed region. It is found that the interface moves down as the Richardson number Ri, based on the conditions of the heated air entering at the top of enclosure, is increased. The stratification can be maintained essentially constant with time. Also, the temperature levels in the two zones can be varied over fairly wide ranges to simulate practical circumstances. The nature of such a stably stratified region is studied, particularly the effect of the inflow and outlet conditions.     

Influence of surface evaporation on stratification in liquid hydrogen tanks of different aspect ratios

The effect of evaporation on stratification in large liquid hydrogen storage tanks of different aspect ratios is computed. A homogeneous two-phase model is used and the continuity, momentum and energy equations for the two phases are solved. Evaporation at the liquid–vapor interface is incorporated through a source term for mass transfer. The amount of stratification is seen to progressively increase as the aspect ratio of the tanks increases. However, the surface evaporation brings down the differences in the amount of stratification with changes of aspect ratio. The predictions for stratification in the pre-evaporation and evaporation phases are discussed.     

A comparison of experimental thermal stratification parameters for an oil/pebble-bed thermal energy storage (TES) system during charging

Six different experimental thermal stratification evaluation parameters during charging for an oil/pebble-bed TES system are presented. The six parameters are the temperature distribution along the height of the storage tank at different time intervals, the charging energy efficiency, the charging exergy efficiency, the stratification number, the Reynolds number and the Richardson number. These parameters are evaluated under six different experimental charging conditions. Temperature distribution along the height of the storage tank at different time intervals and the stratification number are parameters found to describe thermal stratification quantitatively adequately. On the other-hand, the charging exergy efficiency and the Reynolds number give important information about describing thermal stratification qualitatively and should be used with care. The charging energy efficiency and the Richardson number have no clear relationship with thermal stratification.     

Assessing the natural ventilation cooling potential of office buildings in different climate zones in China

This paper presents an investigation of the natural ventilation cooling potential (NVCP) of office buildings in the five generally recognised climate zones in China using the Thermal Resistance Ventilation (TRV) model, which is a simplified, coupled, thermal and airflow model. The acceptable operative temperature for naturally conditioned space supplied by the ASHARE Standard 55-2004 has been used for the comfort temperature setting. Dynamic simulations for a typical office room in the five representative cities, which are Harbin, Beijing, Shanghai, Kunming and Guangzhou, have been carried out. The study demonstrates that the NVCP depends on the multiple impacts of climate, the building's thermal characteristics, internal gains, ventilation profiles and regimes. The work shows how the simplified method can be used to generate detailed, indoor, operative temperature data based on the various building conditions and control profiles which are used to investigate the NVCP at the strategic design stage. The simulation results presented in this paper can be used as a reference guideline for natural ventilation design in China.     

Motion trajectory prediction model of hydrogen leak and diffusion in a stable thermally-stratified environment

The motion trajectory of hydrogen leakage is an essential safe issue for the application of hydrogen energy. A dimensionless fast-running motion trajectory prediction model is proposed to predict the dispersion characteristics of the buoyant jet of hydrogen leakage for the accident. The impact of different leakage angles, leakage velocity and thermal stratification of ambient air on hydrogen leakage behavior was analyzed. The new developed model was verified by experimental results in literatures. Leakage hydrogen can flow upwards freely in a uniform environment. However, it shows an oscillating trajectory at a certain height in a thermally stratified environment, which is so called “locking phenomenon”. The trajectory of hydrogen leakage is upward and hydrogen gathers at the top of the space to form stratification in a uniform environment, while the hydrogen leakage shows an oscillating trajectory at a certain height in a thermal stratification environment. With the increase of Froude number Fr, it shows that the stable height and maximum height of the leakage airflow have a trend of rising first and then falling in a thermally stratified environment. The findings are expected to give guidance in real-world situations, for example, a larger Fr value and a larger temperature gradient can lead to a decrease in the stable height in the thermally stratified environment. It is found that the fitting of the stable height with different temperature gradients satisfies the power function relationship. This work is expected to be helpful for reducing hydrogen leakage accumulation and explosion risk.     

Review of Molten-Salt Thermocline Tank Modeling for Solar Thermal Energy Storage

Molten-salt thermocline tanks are a low-cost option for thermal energy storage in concentrating solar power systems. A review of previous experimental and numerical thermocline tank studies is performed to identify key issues associated with tank design and performance. Published models have shown that tank discharge performance improves with both larger tank height and smaller internal filler diameter due to increased thermal stratification and sustained outflow of molten salt with high thermal quality. For well-insulated (adiabatic) tanks, low molten-salt flow rates reduce the axial extent of the heat-exchange region and increase discharge efficiency. Under nonadiabatic conditions, low flow rates become detrimental to stratification due to the development of fluid recirculation zones inside the tank. For such tanks, higher flow rates reduce molten-salt residence time inside the tank and improve discharge efficiency. Despite the economic advantages of a thermocline tank, thermal ratcheting of the tank wall remains a significant design concern. The potential for thermal ratcheting is reduced through the inclusion of an internal thermal insulation layer between the molten salt and tank wall to diminish temperature oscillations along the tank wall. Future research directions are also pointed out, including combined analyses that consider the solar receiver and power generation blocks as well as optimization between performance and economic considerations.     

An investigation of a nonthermally insulated mixing ventilated office room for partitioning in hot and dry climate by simulation approach

Due to the recent situation, especially since the beginning of the spread of COVID-19, work in administrative or service offices under conditions of social divergence and the working environment has become conditional on the ventilation system; thus, enhancing ventilation's function to minimize the spread of infections, and impurities to larger areas or limiting its transition from one employee to another who work in the same place for long hours. A nonisothermal office room, designed with a mixing ventilation system has been simulated by using AIRPAK3.0.16 software, and RNG k–$\epsilon$ as a turbulence model, where the adoption of partitions in such rooms was studied and its effect on thermal employee's comfort and quality of fresh air, by monitoring the air diffusion performance index, prediction of the mean vote, and percentage of predicted dissatisfied. The main findings were that partitions are seen as a technique of preventing and protecting against the spread of pollutants and illnesses since they prevent contaminants from reaching the breathing zones of the office's inhabitants. It was noted that the change in the height of the partition with an increase of 10% of the room's height as a gap, gives a more acceptance value of the heat removal efficiency and ventilation rate.     

Double-diffusive flow and heat transfer for a cylindrical source submerged in a salt-stratified solution

An experimental study has been performed to delineate the nature of double-diffusive flows induced by a heated horizontal cylinder submerged in a salt-stratified solution. Flow conditions depend strongly on a stratification parameter, N, which measures the initial degree of stable solute stratification relative to destabilizing thermal effects. For large N flow is initially confined to multiple recirculating fluid layers which exist around and above the heat source and are separated by diffusive interfaces. With increasing time the interfaces are eroded, causing merger of the convecting fluid layers and envelopment of the heat source by a single layer. With decreasing N, or in the later stages of an experiment for large N, conditions are strongly influenced by plume development above the cylinder. Flow in the convection layer about the cylinder is driven by the plume, which also enhances degradation of the diffusive interface separating the layer from the overlying fluid. For all values of N, steady-state conditions are only reached after the plume penetrates to the free surface and achievement of a uniform salt concentration eliminates the existence of double-diffusive effects. For a prescribed time the average cylinder Nusselt number decreases with increasing N. However, irrespective of N the Nusselt number increases with time to the limiting value associated with free convection in an unstratified, infinite medium.