Ventilated Cavity Research Articles

Numerical Investigation of Open Cavities with Parallel Insulated Baffles

This research reports the outcome of a numerical investigation of convection in ventilation square cavities contains parallel insulated baffles. The left and right walls of the cavity are kept at the high temperature. Whereas the top, bottom cavity walls, parallel baffles are adiabatic. The opening slots are positioned at the top, bottom corners of the hot vertical walls. The governing Navier-Stokes equations are formulated in the form of vorticity- stream functions. The finite difference method is used to find the values of the primitive variables. The effects of baffles size (Sb − 0.25, 0.50, 0.75), 3 various positions of the parallel baffle, Rayleigh number (103 − 106), Reynolds number (30, 300, 600) are discussed with the flow fields, isotherms, and Nusselt number. It is found that the behavior of ventilation cavities does not only depend on the size of the baffles and its positions. It highly depends on the configuration of the ventilation cavity too. Further, the flow fields are restricted by the largest baffles size of Sb = 0.75.

Numerical simulation of the CO2 diffusion effect on low turbulent mixed convection in a ventilated room heated by the bottom

A double-diffusive mixed convection within low turbulent regime in a ventilated cavities filled with an air CO2 mixture and heated from below has been numerically investigated. The lower wall was sustained at a uniform temperature and CO2 concentration. The vertical and upper walls were kept at external temperature and CO2 concentration. To analyze the behavior of flow, the ventilation effectiveness for temperature distribution and removal of CO2 contaminant, four configurations were dealt. These differ from each other by the location of the mixture inlet and outlet gaps. Likewise, three CO2 concentrations were considered (103, 2?103, and 3?103 ppm) to investigate the influence of the CO2 diffusion on the ventilation effectiveness. The numerical simulations were performed by considering closed Reynolds averaged-Navier-Stokes equations using the Reynolds-normalization group k-? model. The governing equations? set was then solved using the finite volume method, in which the pressure-velocity coupling was handled using the SIMPLEC algorithm. Validation of the numerical model was achieved by comparing our results with available experimental data. The obtained results indicate that the CO2 diffusion effect on the air movement and the ventilation effectiveness for temperature distribution can be neglected in the present study. However, the CO2 diffusion remains a key parameter in terms of indoor air quality index. Also, it was found that one of the studied configurations provides a better ventilation effectiveness to remove heat and CO2 contaminant, and insures a homogeneous temperature and CO2 concentration in the occupied zone. The three other configurations maintain an acceptable level of heat and can be used in temperate climate to ensure good indoor air quality.

Thermal magneto-hydrodynamics in a ventilated porous enclosure

The present study examines mixed convection in a ventilated cavity saturated with porous substance and heated at the right-bottom and top-left corners under the influence of uniform magnetic field applied externally. The analysis is carried out numerically using in-house CFD code. Five different heating configurations are analyzed. The effects of pertinent parameters such as Reynolds number (Re = 1–1000), Richardson number (Ri = 0.1–100), Darcy number (Da = 10−7–10−3), Hartmann number (Ha = 1–100), angle of magnetic field ( $$ \gamma $$ = 0–180°) and porosity (e = 0.1–1.0), on the flow and temperature fields are examined for the search of the appropriate configuration yielding optimum heat transfer rate. Furthermore, heat transfer characterization at different heating configurations and parametric combinations is assessed.

Experimental investigation on ventilated cavity flow of a model ship

A ventilated cavity model ship is investigated in a calm water towing tank under a moderate Froude number range. The gas ventilation rate required for cavity from early formation to complete coverage is explored, as well as the cavity morphology, closure modes, shedding features, and cavity growth characteristics. Several flow patterns of the cavity are classified, i.e., discrete foamy flow and initial coalescing cavity, corresponding to the formation stage; jet growth cavity(JGC), bifurcation growth cavity(BGC), meniscus growth cavity(MGC), and waterfall growth cavity(WGC), corresponding to the logarithmic growth stage; and bottle neck stable cavity, corresponding to the maturity stage. The shedding features of small periodic oscillation shedding, group rips evolution shedding, and wave pinch-off shedding are observed corresponding to JGC, BGC, and MGC and WGC, respectively. Three hypothetical mechanisms, rectangular jet dominant, pressure difference dominant, and wave pinch-off dominant, are proposed to be responsible for the formation of JGC, BGC and MGC, and WGC, respectively. A logarithmic mathematical model is related to cavity length and ventilation rate. The dependence of gas demand required to establish a complete coverage cavity on Froude number (0.37–1.10, ship length based) is more significant than Reynolds number (5.95 × 106–1.78 × 107, ship length based).

Impact of CFD simulation parameters in prediction of ventilated flow on a surface-piercing hydrofoil

ABSTRACT This article discusses numerical prediction of ventilated flows around the surface-piercing hydrofoil. A ventilated cavity is developed on the hydrofoil surface and the growth of the cavity is uncontrolled, which can have harmful effects as it leads to changes in the lift and unexpected loads. The occurrence of ventilation is computed by a Reynolds-Averaged Navier–Stokes solver based on the Finite Volume Method and the Volume of Fluid free surface capturing. The impact of numerical parameters on the ventilation is investigated by testing various momentum, gradient, and interface-capturing schemes, turbulence intesities, acceleration and time stepping scenarios, for 10 Froude numbers and 2 angles of attack. The analysis shows that simulations are consistent and comparable with the experimental data, if proper free surface capturing scheme and very low Courant numbers are used.

Influence of the Lorentz force on the ventilation cavity having a centrally placed heated baffle filled with the Cu − Al2O3 − H2O hybrid nanofluid

The objective of this paper is to examine the natural convection of the ventilation cavity filled with the Cu − Al2O3 − H2O hybrid nanofluid influenced by the effects of the magnetic field. The vertical heated baffle is fixed at the center of the ventilation cavity. The cavity walls are insulted. The opening ports are positioned diagonally at the vertical walls. The finite difference method is employed to solve the governing Navier stokes equations numerically. The computations are done with the Rayleigh number (104 − 106), Reynolds number (20,200,400), Hartmann number (0,20,50), nanoparticle solid volume fraction (0.0,0.03,0.06) and the width of the openings (0.10,0.20,0.30). The computed results are explained in the form of streamlines, isotherms and Nusselt numbers. The results show that the augmentation percentage of the hybrid nanofluid solid volume fraction enhances the heat transfer rate inside the cavity. Although, its performance depends on the other parameters too.

A new double-skin façade system integrated with TiO2 plates for decomposing BTEX

This study proposes a novel double-skin façade integrated with TiO2 plates (T-DSF) for decomposing ambient air Benzene, Toluene, Ethylbenzene, and O-xylenes (BTEX) by utilizing the photocatalytic oxidation technique under natural sunlight. The performance of photocatalytic decomposition on the TiO2 plates was quantitatively compared using the predicted age of air and the amount of BTEX processed. Eighteen cases, i.e., 3 placement strategies of TiO2 plates by 6 airflow gap sizes, were evaluated using a validated computational fluid dynamics (CFD) model to design an optimal T-DSF. The study reveals that the designed T-DSF with 3-m wall height, 1-m wall width, 0.3-m cavity depth, and 0.1-m inlet and outlet height, in which the TiO2 plates are staggered on the external and internal glasses with a 0.02-m airflow gap, can clean about 77 m3/day of ambient air with 20 ppb BTEX (annual average) in Boulder, CO. The sensitivity analysis of solar radiation intensity was studied, which indicates that this system has a stable performance for decomposing BTEX under different solar radiations. Furthermore, the potential impact of particulate matter from ambient air on the photocatalytic oxidation performance was investigated, which was neglectable. This study presents a promising and passive design that can effectively treat BTEX in the atmosphere by taking advantage of UV radiation in natural sunlight and solar-driven natural convection. The concept can be applied in other ventilated cavity wall systems such as the Trombe wall while the key parameters need further fine-tuning.

Physical and numerical study on unsteady shedding behaviors of ventilated partial cavitating flow around an axisymmetric body

The objective is to investigate the unsteady ventilated partial cavitating flow characteristics with focus on the unsteady shedding behaviors via combined experimental and numerical methods. Experimental results are presented for an axisymmetric body in a water tunnel with the flow pattern recorded by a high speed camera. The numerical simulation is performed with a filter-based turbulence model. Good agreement can be obtained between the experimental and numerical results. The ventilated cavity around the axisymmetric body experiences the rapid growth stage, the growth with small pulsation stage and the periodic shedding stage. The typical ventilated cavitating flow patterns at different angles of attack have also been studied to further investigate the effect of the asymmetry on the unsteady shedding behaviors of ventilated partial cavitating flow. With the increase of the angle of attack, the ventilated cavity develops asymmetrically and the non-transparent gas-liquid mixture region is gradually concentrated on the opposing stream surface along with the transparent cavity region increasing on the confronted stream surface.

Hygrothermal Behaviour of Ventilation Cavities in Highly Insulated Envelopes

This article presents long-term experimental studies on the moisture safety in the ventilation cavities of highly insulated (HI) structures. The tested HI-walls had thermal transmittances of 0.11-0.13 W/m2K. A wall with a thermal transmittance of 0.23 W/m2K represented the baseline wall in the test. In addition to walls, an HI-roof of a newly built house with a U-value of 0.08 W/m2K was measured. The results indicate that, in the ventilation cavity, the relative humidity of an HI-wall exceeds 1-7% of the humidity measured from the baseline wall during winter, which coincides with the 0.4-1.5ºC lower temperatures observed in the HI-walls. The mold risk in the ventilation cavities of the walls is low, as the value of the mold index (MI) remains below one, which indicates small amounts of microscopic mold only on surfaces. However, at the bottom of the cavity, the MI value reaches 1.4 due to lower temperatures. In the HI-roof, the MI values are between 1.0 and 2.0 in the middle of the cavity in winter. The reasons for the higher mold risk of the roof are the humid weather, the built-in moisture of the roof and the low heat flux from inside. The study confirms that, in the future, warmer weather and increased humidity can increase moisture risks in the ventilation cavities. The results support the use of materials that are more resistant to mold in the outer parts of structures.

Three-dimensional numerical study of mixed convection within a ventilated cavity (Shape ‘ L ‘) crossed by a nanofluid under the effect of a magnetic field

The present work is dedicated to the three-dimensional numerical study of mixed convection heat transfer, taking place within a ventilated cavity (of shape L) crossed by Cu-water nanofluid. The enclosure is subjected to the action of a magnetic field. The ventilation is assured by two openings of the same size. The cold flow enters by an opening practiced at the top of the left wall, and exits by another opening practiced at the bottom of the right vertical wall. All the cavity walls are maintained at the same temperature, superior to that of the entering flow, except the side walls which are considered as adiabatic. The control parameters are: the Reynolds number and the Hartmann number as well as the nanoparticles volume fraction.

Numerical Study of the Distribution of Temperatures and Relative Humidity in a Ventilated Room Located in Warm Weather

In the present study, an analysis of the heat and mass transfer in a ventilated cavity in a warm climate zone was carried out to analyze, among others, the temperatures and percentage of relative humidity (RH). The governin... | Find, read and cite all the research you need on Tech Science Press

Effects of orientation of the centrally placed heated baffle in an alternative configured ventilation cavity

The current study is carried out to investigate the natural convection of enclosures contains a horizontal/vertical heated baffle at its center with inlet and outlet ports on vertical walls. The vertical walls of the cavity are cold and the horizontal walls are insulated. The working medium of fluid is air with the Prandtl number 0.71. The governing equations of the problem are converted into the vorticity−stream function formulation. The finite-difference method is adopted to solve these equations. The calculations were performed for various Rayleigh numbers $$(10^3 - 10^6)$$ and Reynolds numbers (10, 100, 500), and the effects of the different aspect ratios ($$A=0.25, 0.5, 1.0, 2.0$$) are tested with the most efficient configured cavity with the baffle. The numerical results are illustrated graphically in the form of streamlines, isotherms, and local and averaged Nusselt numbers. The results show that the BB configuration cavity with both orientations of the heated baffle performs better in optimizing the heat inside the cavity. The ventilated cavity with vertical baffle transfers heat more rapidly than the horizontal baffle.

Three-dimensional mixed convection heat transfer in a partially heated ventilated cavity

The 3-D mixed convective heat transfer inside a ventilated cavity partially heated is studied numerically by using control volume method. The heating square portion similar to the integrated electronic devices is placed on the left vertical wall of the enclosure. The right vertical wall is maintained at ambient temperature and all other walls are adiabatic. The results are presented in terms of flow structures, temperature distribution, and global average Nusselt number for various combinations of thermal controlling parameters, namely, the Richardson number (0 ? Ri ? 10), the Reynolds number (10 ? Ri ? 200), the heating section dimension (0. ? ? ? 0.7), and the relative height of the openings B = h/L = 1/8. It is found that for the low Reynolds number the heat transfer process is carried out only by conduction. On the other hand, the highest thermal performance is achieved by reducing the heating section dimension.

Lightweight timber-framed wall and impact of ventilated cladding on the possibility of reducing summer overheating in Central Europe

The climate change creates specific problems in the countries, where it was not so hot during the summer periods in the past. In the area of building envelopes, use of more environmental friendly materials comes to the foreground. Having still more strictly requirements in the thermal protection of buildings, use of lightweight wooden walls is increasing. The ratio between load carrying and insulating parts changed. This is also the advantage of timber framed walls. These walls have also some negatives, or create some problems, such as low thermal capacity. In the Central Europe, the indoor summer overheating started to be a serious problem. In this paper, impact of ventilated cladding on the timber frame wooden wall will be analyzed. One composition, with east and south orientation were evaluated. This wall is a part of the experimental measurement, compared with another ETICS type of outdoor finishing layer. Impact of the ventilated air cavity behind the cladding on the thermal regime of the wall is stated.

Curtain Wall with Solar Preheating of Ventilation Air. Full Scale Experimental Assessment

Heating load in Commercial buildings is highly related with ventilation systems, while at the same time local discomfort in the vicinity of glass walls occurs due to overheating. In this paper, a novel double envelope curtain wall is presented, which extracts heat from the façade by means of a ventilated cavity which is then incorporated to the ventilation air intake. A substantial reduction of heating loads is achieved. Whenever solar gains are not sought, a bypass element allows the natural ventilation of this air cavity, acting as a ventilated façade. An integrated control system with embedded electronics and actuators allows for a smart control of the system. The system is designed for integration with existing rooftop ventilation systems. Design considerations are discussed, and the outcomes of a full-scale experiment conducted in Bilbao (Spain) along 2019 presented.

Experimental Validation of a Numerical Model of a Ventilated Façade with Horizontal and Vertical Open Joints

Commercial and residential building is one of the four major final energy consumption and end-use sectors. In this sector, cooling loads represent an important part of the energy consumption, and therefore, they must be minimized, improving the energy efficiency of buildings. Ventilated façades are one of the most widely used passive elements that are integrated into buildings, precisely with the aim of reducing these loads. This reduction is due to the airflow induced in the air cavity by the buoyancy forces, when the solar radiation heats the outer layer of the façade. In the open joint ventilated facades (OJVF), ventilation is attained through the open joints between the panels composing the outer layer. Despite the steadily growing research in the characterization of this type of system, few studies combine the numerical modelling of OJVF with experimental results for the assessment of the airflow in the ventilated cavities. This paper experimentally validates a numerical simulation model of an OJVF. Firstly, the façade performance has been experimentally assessed in a laboratory model determining the temperatures in the panels and air gap and measuring the flow field at the gap using particle image velocimetry (PIV) techniques. Secondly, a numerical model has been developed using advanced Computational Fluid Dynamics (CFD) simulation tools. Finally, an experimental validation of the numerical model has been done. Experimental and numerical results are compared in different planes inside the ventilated cavity. The discrete ordinates (DO) radiation model and the k-ε renormalisation group (RNG) turbulence model better adjust the simulated results to the experimental ones.

The hydroelastic response of a surface-piercing hydrofoil in multiphase flows. Part 2. Modal parameters and generalized fluid forces

The fluid–structure interactions (FSI) of compliant lifting surfaces is complicated by free-surface and multiphase flows such as cavitation and ventilation. This paper describes the dynamic FSI response of a flexible surface-piercing hydrofoil in dry, wetted, ventilating and cavitating conditions. Experimental modal analysis is used to quantify the resonant frequencies and damping ratios of the fluid–structure system in each flow regime. The generalized hydrodynamic stiffness, fluid damping and fluid added mass are also determined as ratios to the corresponding structural modal forces. Added mass increases with increasing partial immersion of the hydrofoil and decreases in the presence of gaseous cavities. In particular, modal frequencies were observed to increase significantly in fully ventilated flow compared to fully wetted flow. The modal frequencies varied non-monotonically with speed in fully wetted flow. Gaseous cavities reduced the modal added mass and reduced the fluid disturbing force. Modal damping increases non-monotonically with increasing immersion depth. Forward speed causes the fluid damping force to increase with an approximately quadratic functional behaviour, consistent with a series expansion of the Morison equation, although damping identification became increasingly difficult at high flow speeds. The results indicate that fluid damping is greater than the associated structural damping in a quiescent liquid, and increasingly so with increasing immersion, suggesting viscous dissipation as a dominant mechanism. A preliminary investigation of modal vibration as a means of controlling the size and stability of ventilated cavities indicates that low-order modes encourage the formation of ventilation, while higher-order modes encourage the washout and elimination of ventilation.

The role of wall layers properties on the thermal performance of ventilated facades: Experimental investigation on narrow-cavity design

In this paper we have investigated how different materials and thermal masses impact on the performance of ventilated facades with narrow cavities, by measuring the variation in terms of heat flows and ventilation efficiency. While geometry has been widely explored, the role of wall composition has received much less attention. To bridge the gap, three real-scale prototypes of ventilated facades were built and tested all over the year on a mock-up in Central Italy: (i) L, with a lightweight external enclosure, as a baseline reference, (ii) IM, with a massive layer enclosed in the gap and (iii) EM, with an external massive cladding. The results demonstrated that the EM solution more effectively mitigated the average surface temperatures (both external and internal), with values of –2 °C and –1 °C in summer and of –3 °C and –0.5 °C in winter, when compared to the L solution. Moreover, in the EM case, the ventilated cavity reduced both the incoming and outgoing heat fluxes, since the outer mass operated as a thermal buffer between the outdoor and the ventilation chamber. Conversely, the presence of an internal mass determined an increase of the heat transfer towards the indoor environment. The position of the thermal mass in the outer layer also increased the air velocity in the gap thus enhancing the stack effect.

Numerical study of the effects of ventilated cavities outlet location on thermal comfort and air quality

PurposeA comprehensive investigation on the outlet air position effects on the thermal comfort and air quality has been achieved. In addition, airflow and temperature distributions in ventilated cavities filled with an air-CO2 mixture with mixed convection are predicted. The airflow enters from the cavity through an opening in the lower side of the left vertical wall and exits through the opening in one wall of the cavity. This paper aims to investigate the outlet location effect, four different placement configurations of output ports are considered. Three of them are placed on the upper side and the fourth on top of the opposite side of the inlet opening. A uniform heat and CO2 contaminant source are applied on the left vertical wall, while the remaining walls are impermeable and adiabatic to heat and solute. The cooling efficiency inside the enclosure and the average fluid temperature are computed for different Reynolds and Rayleigh numbers to find the most suitable fluid outlet position that ensures indoor comfortable conditions while effectively removing heat and the contaminant. This is demonstrated by three relevant indices, namely, the effectiveness for heat removal, the contaminant removal and the index of indoor air quality.Design/methodology/approachThe simulations were performed via the finite-volume scSTREAM CFD solver V11. Three different values of CO2 amount are considered, namely, 103, 2 × 103 and 3 × 103 ppm, the Reynolds number being in the range 100 ≤ Re ≤ 800.FindingsBased on the findings obtained, it is the configuration whose air outlet is placed near the heat source and the contaminant, which provides a better air distribution and a ventilation efficiency compared to the others ventilation strategies.Originality/valueThe studies on heat and mass transfers by natural and forced convection in ventilated cavities remain a fruitful research topic. Thereby, such a study deals with different ventilation strategies through cavities containing an air-CO2 mixture subjected to a mixed regime. In particular, the air inlet velocity and contaminant sources’ effects on thermal comfort and air quality have been investigated.

Numerical prediction of surface radiation effect on thermal comfort and indoor air quality in a ventilated cavity heated from below

This paper presents an investigation of the influence of surface radiation on indoor environmental parameters such as thermal comfort and indoor air quality. To achieve this objective, coupled heat transfer by mixed convection and radiation was numerically carried out inside a ventilated cavity. Air-CO2 mixture was considered as the working fluid. Uniform heat and CO2-concentration were applied at the bottom wall of the cavity. The boundary conditions of other walls were fixed at the external conditions. The same emissivity value was considered for all interior surfaces. An external fresh air enters at the cavity through an opening located on the top of the left vertical wall and exits from another one located at the bottom of the opposite wall. The mass, momentum, energy and chemical specie equations, coupled with the RNG k-ε turbulence model were solved via finite volume method. The obtained results indicate that the surface radiation presents slight effect on thermal comfort indexes, while the increasing of Rayleigh number enhances them. A good indoor air quality is insured inside the studied cavity.