Ventilated Cavity Research Articles

Mixed convection and surface radiation in a ventilated cavity containing two heat-generating solid bodies

This work aims to numerically study the mixed laminar convection and surface radiation in a square air-ventilated cavity (Pr=0.71). Inside this cavity, two square bodies are placed at the same height and generate the same amount of heat. This model can be applied to cooling electronic components. The air enters the cavity from an inlet port at the bottom of the left vertical wall and exits in the outlet port at the top of the other vertical wall. The left wall of the cavity is maintained at a constant temperature TC, while the other walls are assumed to be adiabatic. The Finite Volume Method (FVM) and the SIMPLE algorithm are used to discretize and solve the governing equations. The effect of Rayleigh and Reynolds numbers and surface emissivity on streamlines, isotherms, maximum temperature, and convective and radiative Nusselt numbers are discussed. The obtained results indicate that the maximum temperature decreases with increasing the Reynolds number. It also observed that the surface radiation has more effect for low values of Re. For engineering purposes, correlations have been developed that give the maximum temperatures for ε=0or1, and different ranges of Reynolds number.

Double Diffusive Convection Heat and Moisture Transfer Inside a Planted Roof Building Under Hot Humid Climate: Case of Lomé City in West Africa

Planted roofs have been investigated as a passive cooling technical for the energy efficiency purpose in buildings. More quantitative data on this topic are required to solve a lack of information for many specific regions. This study is focused to a numerical investigation of the thermal comfort inside a green roof rectangular ventilated cavity in a hot and humid climate as the one of Lomé in west Africa. The left vertical is heated and partly saturated with water to provide humidification of the indoor air. Transfer dimensionless equations are solved using an implicit finite difference scheme, Thomas algorithm and the Gauss Seidel iterative method. We analyze the effects of inlet airflow on the thermal process inside the ventilated and planted enclosure have been investigated. The comfort temperature range deduced from data is: 25°C< Tc < 27°, that of the indoor air humidity is: 49% < Hr <60%. The different ranges obtained are significant and lead to improve inside thermal comfort. The solar flux 350 W.m-2, the average value in case of Lomé city, was used to establish a heat transfer correlation to predict heat transfer through the roof with a relative error not exceeding 4%. This model can be very useful for engineers in the design and optimization stage of a green roof in practical buildings.

HEAT-TRANSFER BEHAVIORS OF A SLIGHTLY HORIZONTAL VENTILATED ROOF UNDER VARIABLE CLIMATIC CONDITIONS

This paper illustrates a numerical study to evaluate thermal and fluid dynamic behaviors of natural convection inside a slight inclined ventilated roof for residential building in Mediterranean area. The system is studied considering a roof with a length equal to 6.0 m and three different inclinations with respect to the horizontal plane (0°, 2°, 5°). The ventilated channel, under the roof, has a distance between the flat plates equal to 10 cm. A two-dimensional natural convection with surface radiation model is considered assuming a turbulent air flow. Different conditions are applied on the top wall and the bottom wall of the roof to simulate summer and winter conditions. Different values of solar heat fluxes are applied on the top wall of the ventilated cavity together with heat transfer with the external ambient. Along the bottom wall there is a heat transfer toward an internal ambient by means a thermal conductance. The problem is solved by means of the commercial code Ansys-Fluent. Results are given in terms of temperature and pressure distributions, air velocity and temperature profiles. The increase in inclination of the pitched roof determines a better performance of the component. In fact, during the summer, the efficiency of the system improves because the high heat flux produces an optimal air-convective condition into the ventilated channel. In the winter, the effect of the ventilated layer is more significant to fulfill optimal thermal and hygrometric conditions.

Measuring the effective thermal resistance of ventilated air-spaces behind common wall assemblies: theoretical uncertainty analysis and recommendations for the hot box method modifications (ASHRAE 1759-RP)

A well-designed ventilated wall that incorporates air-space behind cladding can reduce energy use for conditioning buildings by adding a thermally resistive layer. The scarcity of thermal performance data and standardized test methods is a particular barrier for practitioners to implement ventilated air gaps behind the facades. The current standardized test method to evaluate the thermal performance of a wall structure, such as ASTM 1363-19, does not consider the effect of the ventilated air-space behind external claddings. Therefore, testing and design recommendations are required to account for the impact of ventilation airflow on the thermal resistance of vertical air-spaces behind common cladding materials. In this study, the theoretical uncertainty analysis of the effective thermal resistance of the ventilated cavity is performed for a steady-state condition. The results revealed that temperature sensors with an absolute uncertainty of ±0.18 (±0.1 °C) and heat flux sensors with a relative uncertainty of 3% or lower are required to capture a wide range of the effective thermal resistance of the ventilated air gap. Thereafter, based on the uncertainty of temperature and heat flux measurements, the modifications of the ASTM C1363-19 test method to account for the airflow effects on the thermal performance of the wall assembly are proposed. A detailed description of the experimental setup is provided, and protocols for collecting, analyzing, and evaluating data are suggested. An example demonstrating how the proposed method can be practically used to measure the thermal resistance of a ventilated air-space is also presented.

Numerical study on the effects of modulated ventilation on unsteady cavity dynamics and noise patterns

Supercavitating flow is accompanied by significant unsteady characteristics, and it is therefore very important to find methods to control this multiphase flow phenomenon. Ventilation is an important method for creating supercavitation and it affects the evolution, load, and noise characteristics. In this paper, cavity flows with and without modulated ventilation (i.e., the imposition of a sinusoidal component on the ventilation rate) are investigated using computational fluid dynamics techniques incorporating large eddy simulation, coupled with the Ffowcs Williams–Hawkings (FW–H) method. The effects of modulated ventilation on cavity shedding, vortex structure, and the noise characteristics of the cavity are compared and analyzed. The results show that modulated ventilation can change the shedding period of the ventilated cavity and can slightly improve its lift and drag performance. It can also promote the formation and growth of hairpin vortices and impose a periodicity on the evolution of the vortex structure. Furthermore, although modulated ventilation cavitation enhances pressure fluctuations near the vent and increases the self-noise of ventilation, it has little impact on far-field noise while reducing the turbulence of the far field, which decreases the total sound pressure level in the wake of the cavitator.

Energy balance of the airflow boundary layer in the brake disc ventilation

Airflow through the brake disc ventilation causes the formation of a boundary layer at the walls. It affects both the dynamic processes related to air exchange in the space between the walls and thermal processes associated with air insulation of the heated surfaces of the ventilation ducts. The present paper aims to develop a model for calculating plane airflow in a ventilation duct in polar coordinates. Using the Navier-Stokes equations and the equations of the energy balance of the airflow boundary layer, we succeeded in determining the elements that affect the intensity of changes in the air masses in the boundary layer and the elements that are responsible for the thermal conductivity of the thermal boundary layer of the airflow. Besides, we obtained an energy balance equation, which takes into account the enthalpy and thermodynamic parameters of the thermal boundary layer, as well as found the possibilities of influencing the heat exchange processes by minimizing factors of the heat-insulating boundary layer. Finally, we specified the dependence of the boundary layer temperature on the temperature of the walls of the brake disc ventilation. The obtained dependences lay the ground for formulating variants of the influence on the heat-insulating boundary layer of the airflow, namely, the design of a forced air supply system at different angles of attack into the ventilation cavity of the brake disc or the manufacture of ventilation ducts with complex geometry.

Numerical investigations into the ventilation elimination mechanism of a surface-piercing hydrofoil

This paper investigates the ventilation elimination mechanisms during the deceleration process of a surface-piercing hydrofoil using the unsteady Reynolds-averaged Navier-Stokes (RANS) method together with a Volume of Fluid (VOF) model. The numerical results are in good agreement with the experimental data. The ventilation elimination mechanism of the surface-piercing hydrofoil is analyzed from the perspectives of the hydrofoil hydrodynamic performance, the ventilated cavity evolution, vortex structures, and re-entrant jets. The results indicate that the ventilation elimination includes three stages, i.e. a decrease in the ventilated cavity, washout, and reattachment. The decrease in the ventilated cavity is due to the hydrofoil speed decrease in the FV flow. Washout is the transition from fully ventilated to partially ventilated flow, and reattachment is the transition from partially ventilated to fully wetted flow. The underwater vortex structures around the surface-piercing hydrofoil are composed of a tip vortex, an unstable vortex induced by the shear layer, and a Karman vortex caused by the vortex shedding from the trailing edge of the hydrofoil. Ventilation stability strongly depends on the re-entrant jet. When Φ (the angle between the flow direction and the closure line of the ventilated cavity) is greater than 45°, the re-entrant jet impinges on the ventilated cavity's leading edge and destabilizes the ventilated cavity.

Numerical modeling and optimization of annual thermal characteristics of an office room with PCM active–passive coupling system

Herein a numerical study was conducted on a PCM coupling room integrated with a PCM active–passive coupling system to optimize it annual thermal characteristics. In detail, four kinds of inorganic composite PCMs were simultaneously introduced into the wall, ventilation cavity and double-layer radiant floor to construct a PCM active–passive coupling system. The numerical model constructed by EnergyPlus was validated based on the experimental result. The simulation results shown that, compared with the room with common radiant floor, the PCM coupling room exhibited a lower annual energy consumption and a higher indoor thermal comfort rate, which was increased by 16.58%. Based on the thermal comfort evaluated by PMV-PPD method and annual energy consumption, a parameter optimization of the double-layer PCM radiant floor was performed, including the PCM thickness, the thermal conductivity and the location of the heat source. It was found that appropriately increasing the total thickness of the PCM layer could effectively enhance its heat storage capacity to improve the indoor temperature control effect. Specially, the total thickness and the thickness of the upper and lower PCM layers had optimal values. Furthermore, when the thermal conductivity of the upper PCM layer was fixed, increasing that of the lower PCM layer could improve the heat storage /release efficiency of the floor, while its enhancement effect gradually decreased. In addition, moving up or down the heat source would reduce the utilization rate of the PCMs. The radiant floor exhibited the optimal overall thermal performance when the heat source was placed between the two PCM layers. Finally, after parameter optimization, the PCM coupling room performed an indoor thermal comfort rate as high as 81.58%, while saving 20% of the electricity cost by utilizing the price difference between peak and valley electricity.

In-situ measurements of the U-value of a ventilated wall assembly

The walls in a building envelope have the largest contact area with the exterior environment, and, therefore, a considerable portion of the thermal energy can be lost through the walls compared to the other parts of the building envelope. For energy-saving purposes, the thermal transmittance of walls is typically limited by building energy performance standards at the national level. However, the presence of a ventilated air-space behind the external cladding, which has variable hydro-dynamic behavior, can differently affect the total thermal transmittance of the entire structure. This paper aims to provide an experimental analysis of the total U-value of a ventilated wall assembly measured in a building prototype following the average and dynamic methods defined by ISO 9869-1. Differences between the calculated theoretical U-value and the measured U-value are compared. The contribution of the thermal resistance of the ventilated air-space in the total thermal transmittance of the wall assembly is also analyzed. The results show that the air movement and the enthalpy change in the ventilated cavity can affect the thermal performance of the wall structure to a certain extent.

Comparing calculation methods of state transfer matrix in Markov chain models for indoor contaminant transport

Fast and accurate prediction of indoor airborne contaminant distribution is of great significance to the safety and health of occupants. Several Markov chain models have been developed and proved to be the potential solutions. However, there is a lack of comparison in terms of accuracy, computing cost, and robustness among these models, which limits their practical application. To this end, this study compared the performance of three Markov chain models, in which the state transfer matrix was calculated based on different principles, i.e., Markov chain model with flux-based method, with Lagrangian tracking, and with set theory approach. The investigation was conducted in a 2D ventilated cavity and a two-zone ventilated chamber. The simulation by Eulerian model for contaminant and experimental data were used as the benchmarks for the 2D and 3D cases, respectively. It is revealed that all three Markov chain models can provide a correct prediction. In the 2D case, the Markov chain model with set theory approach is the most accurate, followed by Lagrangian tracking. In the 3D case, the accuracy of Markov chain models with flux-based method and Lagrangian tracking is comparable. The Markov chain model with Lagrangian tracking is the fastest, and the time step size in this model can be relatively large. Finally, the selection guideline is given for the application of Markov chain models in different scenarios.

Numerical investigation of convective heat transfer in a rectangular vented cavity with two outlets and cold partitions

The convective flow in a ventilated cavity having two outlets and one inlet is numerically investigated. Introducing cold partitions inside the cavity modifies heat and fluid flow characteristics. It is analyzed by placing a single and double partition inside the cavity. Three configurations of the rectangular cavity are analyzed in the laminar flow range, with Rayleigh numbers varying between 103 and 106. The Navier Stokes equation, which governs the flow, is solved using the finite difference method. The medium of flow is considered as air with Prandtl number 0.71. The result shows that the fluid entering the inlet splits into two to enter the two outlets, and this cold air occupies most of the central place of the cavity at higher Ra when there is no partition in the enclosure. With the introduction of cold sections, a part of fluid gets attracted towards it and creates fluid circulation. It decreases the velocity of the fluid at the bottom of the cavity at the midsection of outlet 2. The fluid velocity at outlet 1 is not much affected. The average heat transferred to the right wall gets significantly decreased due to the cold partitions at lower Rayleigh numbers, but there is an increase in the overall average Nusselt number of all outer walls. A correlation equation that gives the relationship between Nusselt number and Rayleigh Numbers is developed. The study presents the variation of isotherms, streamlines, velocity, rate of heat transfer and correlation of Nu for the considered problem.

Physical and numerical study on the transition of gas leakage regime of ventilated cavitating flow

The objective of this paper is mainly to investigate the ventilated cavitating flow around an axisymmetric body with a focus on the gas leakage mechanisms via combining experimental and numerical methods. A high-speed camera technique is used to record the ventilated cavitating flow patterns. The numerical simulation is performed with a free surface model and Filter-based turbulence model (FBM). Good agreement can be obtained between the experimental and numerical results. In the formation of a ventilated cavity, there are three typical gas leakage types with different Froude numbers Fr and gas entrainment coefficients CQ, including RJ closure mode with gas leakage of toroidal vortex shedding (TVS-RJ), TV closure mode with gas leakage of twin-vortex tube entrainment (TVTE-TV), and the coexistence of toroidal vortex shedding and twin-vortex tube entrainment (TVS-TVTE). The internal flow characteristics revealed three distinct regions with the ventilated cavity, including the ventilation influence region, the internal shear layer and the reverse flow region. With the increase of CQ, the incline angle of the cavity bottom surface α gradually decreases. When the incline angle α decrease to a critical value of ∼17.1°, the flow separation disappears completely and the flow regime migrates completely to TV cavity from RJ cavity.

Study on ventilated cavity uncertainty of the vehicle under stochastic conditions based on the Monte Carlo method

Ventilated cavities play an important role during water-exit of underwater vehicles. However, the development of ventilated cavities suffers various uncertainties, which lead to remarkable uncertainty in the cavity shape and internal fluid field structure. By constructing the stochastic sample space based on the Monte Carlo method, this paper investigates the uncertainty of the ventilated cavity under stochastic conditions (incoming flow velocity, ventilation flow rate, and ambient pressure). The results show that the covering area of the ventilated cavity can provide good pressure-equalizing robustness under stochastic conditions. The cavity length and re-entrant pressure at the cavity tail are sensitive to stochastic conditions. The vortex structure inside the cavity expands with the development of the re-entrant jet and gradually transforms from multiple pairs of vortices to a single large-scale vortex. The high standard deviation of the phase volume fraction also transforms from the middle of the vortex pair to the tail of the large-scale vortex. The structure and strength of the vortex in the air phase region are not sensitive to stochastic conditions, but the re-entrant jet at the cavity tail is more sensitive to stochastic conditions, which lead to an increase in the standard deviation of pressure at the cavity tail.

Unsteady behavior of ventilated cavitating flows around an axisymmetric body

The objective of this paper is mainly to investigate the ventilated cavitating flow characteristics around an axisymmetric body with a focus on three-dimensional unsteady behaviors. A high-speed camera technique is used to record the ventilated cavitating flow patterns. A homogeneous free surface model coupled with Filter-based turbulence model is used to simulate the time-evolution process of the unsteady cavitating flows. Numerical results show a reasonable agreement with the experimental data. The evolution of the ventilated cavity quasi-periodic unsteady shedding is well captured and discussed. The asymmetric and three-dimensional unsteady behaviors are impressive in the ventilated cavitating flow. Further analysis demonstrates that the re-entrant jet which originates from different circumferential positions at the closure region of the cavity moves upstream with different speeds. Lagrangian coherent structures (LCS) methods are applied to reveal the influence of re-entrant jet on the U-type cavity shedding and it shows that there is a close relationship between unsteady behaviors and the re-entrant jet flow motion consisting of moving upstream and circumferential. In addition, the effect of asymmetry on unsteady shedding behaviors of ventilated cavitating flow at different angles of attack have also been investigated combined with LCS methods.

Experimental and numerical study of mixed convection with surface radiation heat transfer in an air-filled ventilated cavity

The present study deals with the experimental and numerical investigation of steady combined laminar mixed convection and surface radiation heat transfer in the air-filled enclosure subjected to a constant heat flux supplied to the vent side of the cavity. Experimental results are presented for heat inputs, Q = 3.19 W–16.71 W, Reynolds number, Re = 4814–166385, and Richardson number, Ri = 0.1–25. In the present study, the numerical simulation is also performed in the two–dimensional vented cavity to enrich the experimental work for aspect ratio, A = 1–5, and surface emissivity, ε = 0.05–0.85. The momentum and energy equations under Boussinesq approximations are solved using a well-known finite-volume method, and numerical code is developed in Fortan 90 for getting all numerical results. Based on the experimental data, correlations are developed for the Nusselt number and maximum temperature of the heated surface.

Effect of Passive Flow Control Devices on Base Pressure for Mach Numbers Between 0.5 and 1.4

Abstract Experimental studies are carried out on an axisymmetric cylindrical base body for six freestream Mach numbers between 0.54 and 1.41. Unsteady pressure is measured on the base surface using high-frequency response Kulite pressure transducers. The effect of passive flow control devices on the mean base pressure and the unsteady characteristics of base pressure have been studied. A blunt base, a conventional cavity device, and three different ventilated cavity devices have been tested along with four different rounded base lip devices. A total of 20 different base geometric modifications are tested at 6 freestream Mach numbers resulting in 120 test cases. The cavity devices improve the base pressure as compared to the blunt base case, particularly for freestream Mach numbers more than 0.98. Among all the cases considered, a maximum increase of 8.6% in the base pressure coefficient is noticed for the normal ventilated cavity device as compared to the blunt base case for freestream Mach number of 1.22. The power spectral density of base pressure fluctuations revealed the dominant peaks on the base surface. The Strouhal number associated with the coherent structures developing in the shear layer varies between 0.2 and 0.27 for the six freestream Mach numbers considered. In the presence of cavity devices, dominant peaks are observed for Strouhal numbers between 1 and 5. The root-mean-square, skewness, kurtosis of the base pressure fluctuations for all the cases are presented. Maximum reduction in base pressure fluctuation is observed for the normal and inclined ventilated cavity device configuration test cases.

Numerical Study on Energy-Saving Performance of a New Type of Phase Change Material Room

The thermal performance of a phase change energy storage building envelope with the ventilated cavity was evaluated. CaCl2·6H2O-Mg(NO3)2·6H2O/expanded graphite (EG) was employed to combined with the building for year-round management. The energy consumption caused by the building under different influence parameters was evaluated numerically. The results indicated that CaCl2·6H2O-8wt %Mg(NO3)2·6H2O/EG should be installed on the south wall for the heating season, while CaCl2·6H2O-2wt %Mg(NO3)2·6H2O/EG should be integrated on the roof for the cooling season. When the air layer was ventilated and the south wall was coated with the solar absorbing coating, the room could save approximately 30% of energy consumption. Moreover, the energy consumption increased with an increase in the air layer thickness, and the air layers played a different role in the building envelope. The optimal value of the flow rate between air layer 2, air layer 3, and the room was 0.09 m3/s. To reduce the energy consumption, the phase change materials (PCMs) with large and small thermal conductivity should be installed in the south wall and roof, respectively. In general, the phase change energy storage building envelope with the ventilated cavity can save energy during the heating and cooling seasons.

THE AERODYNANIIC CALCULATION OF RAILROAD TUNNELS LINE OF MARABDA-AKHALQALAQI

The provided air expenses in tunnel are determined with the aerodynamic calculation of train motion pistol effect and after movement effect. It is ascertained, that for the tunnel conditions air expenses that are processed with pistol effect are approximately 2/3 removed towards train motion and 1/3 flows орposite to the free space between train and tunnel strengthening. The last one provokes air turbulence and represents common aerodynamic resistance stand face. Despite this, all galleries of Marabda-Akhalqalaqi train main lines can be aerated on expense of train pistol effect with aroused natural pressure. The use of his pressure could be more effective if every chamber and niche will be equipped with ventilation cavity on the both sides of tunnel. The every cavity section area should be 5.6 m2. The noted measures will shorten aerodynamic resistance a rouse by air flowing in distances and would be more outhunted by calculation of railroad tunnels line of Marabda—Akhalqalaqi.

Numerical investigation on fluid dynamic characteristics around shoulder ventilation of submarine-launched vehicle

In order to study the influence of the ventilating cavitation flow at the shoulder of a submerged-launched vehicle on the surface hydrodynamic characteristics, a three-dimensional potential model for the shoulder ventilation of the vehicle was established based on the homogeneous multiphase flow theory, standard RNG k-ε model, Singhal cavitation model and overlapping grid technology, and the numerical simulation of the unsteady evolution process of the ventilated cavitation flow was carried out, and the cavitation flow morphology evolution, surface pressure distribution and resistance characteristics under different ventilation rates were compared. The results showed that the thickness and length of the ventilated cavitation flow in the early stage of fusion continue increased with the increasing of ventilation volume, and its thickness and length changed slightly in the later stage; when the exhaust position did not change and the ventilation volume was within a certain range, the differential pressure resistance coefficient and viscous resistance coefficient decreased with the increasing of internal pressure of the ventilated cavity.

Numerical modeling and investigation of the effect of internal waves on the dynamic behavior of an asymmetric ventilated supercavity

In this paper, a realizable two-layer k−ε turbulence model and the Schnerr-Sauer cavitation model are used to simulate dynamic behavior of asymmetric ventilated supercavies while considering the effects of internal wave s. The effectiveness of the numerical method is verified via comparison with the available experimental data and the analytical solution of the internal wave velocity. The results show that the presence of internal waves affects the supercavity closure patterns and shedding periods. Compared to a system with no internal waves, the cavity shedding and pressure pulsation intensity increase and the shedding period is closely related to that of the internal wave. Internal waves also play an important role in the distribution of ventilated cavitation patterns inside the cavity. The local cavitation number is shown to reflect the pressure change during cavity evolution and indicates that the internal wave affects the flow field inside the cavity. In addition, the effects of internal waves of various amplitudes on cavitator hydrodynamic performance are studied. As the internal wave amplitude increases, the cavity pulsation evolution becomes more intense. Furthermore, the ventilated cavity intensity, lift, and drag are closely related to the amplitude of internal waves.