Vertical Rectangular Duct Research Articles

Effect of vent size on vented H2/N2/air deflagration

In this work, the coupling of explosion venting (vent size) and inerting (N2) on H2/air deflagration was investigated. Experiments were carried out in a vertical rectangular duct with an upper opening at an initial temperature and pressure of 280 K and 101 kPa, and a vent coefficient Kv was employed to replace the vent size to elucidate its effect on pressure buildup and flame propagation during H2/N2/air deflagration. In the present study, the internal pressure profile is monitored by three pressure sensors, PS1-3, which are installed at the bottom, center, and top of the duct, respectively, and the external pressure profile is obtained by PS4. The results show that the maximum internal overpressure (pmax) recorded by PS1-3 all increase with increasing Kv, but pmax obtained from PS2 and PS3 tend to increase linearly. For a given Kv, the maximum and minimum amplitudes of pmax are measured by PS1 and PS3, respectively. Specifically, the greater the distance between the pressure sensor and the upper opening, the greater the amplitude of pmax. The relationship between the maximum reduced explosion pressure (pred) and Kv is investigated, where pred is defined as the highest pmax for a specific Kv. As Kv increases from 2.2 to 11.9, pred increases from 26.25 kPa to 88 kPa. The maximum external flame velocity (Vext) increases from 83 m/s to 454 m/s with increasing Kv from 2.2 to 11.9. The amplitude of the maximum external overpressure (pext) first increases and decreases with an increase in Kv from 2.2 to 11.9. The formation time (Δt) of pext decreases and then increases and finally decreases with increasing Kv from 2.2 to 11.9. When Kv is increased from 2.2 to 4.1, all external fireballs are presented as mushroom shapes, but the external fireballs are gradually transformed into jet shapes for Kv ≥7.8. Two pressure oscillations, including Helmholtz oscillations and acoustic oscillations, are found. Acoustic oscillations are found in all tests, but Helmholtz oscillations are only observed for Kv ≤ 4.1.

Novel Numerical Investigations of Some Problems Based on the Darcy–Forchheimer Model and Heat Transfer

In this study, we introduced a new type of basis function and subsequently a Chebyshev delta shaped collocation method (CDSC). We then use this method to numerically investigate both the natural convective flow and heat transfer of nanofluids in a vertical rectangular duct on the basis of a Darcy–Brinkman–Forchheimer model and the electroosmosis-modulated Darcy–Forchheimer flow of Casson nanofluid over stretching sheets with Newtonian heating problems. The approximate solution is represented in terms of Chebyshev delta shaped basis functions. Novel error estimates for interpolating polynomials are derived. Computational experiments were carried out to corroborate the theoretical results and to compare the present method with the existing Chebyshev pseudospectral method. To demonstrate our proposed approach, we also compared the numerical solutions with analytic solutions of the Poisson equation. Computer simulations show that the proposed method is computationally cheap, fast, and spectrally accurate and more importantly the obtained approximate solution can easily be used by researchers in this field.

A Comparative Study on MHD Forced Convective Flow of Different Nanofluids with Water (H2O) as Base Fluid in a Vertical Rectangular Duct

A Comparative Study on MHD Forced Convective Flow of Different Nanofluids with Water (H2O) as Base Fluid in a Vertical Rectangular Duct

Molecular Interaction and Magnetic Dipole Effects on Fully Developed Nanofluid Flowing via a Vertical Duct Applying Finite Volume Methodology

Interpreting the complex interaction of nanostructured fluid flow with a dipole in a duct, with peripherally uniform temperature distribution, is the main focus of the current work. This paper also sheds light on the changes in the Nusselt number, temperature profiles, and velocity distributions for the fully developed nanofluid flow in a vertical rectangular duct due to a dipole placed near a corner of the duct. A finite volume approach has been incorporated for the numerical study of the problem. It is interesting to note the unusually lower values of the Nusselt number for the higher values of the ratio Gr/Re. Due to the nanostructure in the fluid, an enhancement in the Nusselt number has been noted, which is strongly supported by the magnetic field caused by the dipole. However, as the duct shape is transformed from rectangular to square, the Nusselt number is reduced remarkably. Further, as the dipole is brought nearer to the duct corner, the Nusselt number increases significantly. On the other hand, the flow reversal in the middle of the duct has been noted at higher values of the ratio Gr/Re. The dipole is noted to have a low impact on the reversal flow as well as on the temperature distribution.

Influence of system buoyancy and hydraulic diameter on turbulent mixed convection in a rectangular duct

We investigated buoyancy-aided, turbulent mixed-convection heat transfer in a vertical rectangular duct. In order to achieve highly buoyant conditions, mass transfer experiments instead of heat transfer ones were performed based on the analogy between heat and mass transfer. Reynolds numbers ranged from 2,700 to 25,450 and Grashof numbers were 3.30 × 105 and 5.15 × 106. The results of turbulent forced-convection experiments were consistent with previous studies. Notably, at the same axial position, local heat transfer was affected by downstream flow for different heated lengths, and heat transfer was affected by the hydraulic diameter. Five distinct flow regimes were observed based on buoyancy coefficients. Particle image velocimetry measurements revealed that variations in local heat transfer were closely related to the amount of turbulence produced, which was affected by differences in velocity between buoyant and forced flows. Turbulent mixed-convection heat-transfer correlations are proposed based on analytical models and parametric studies of the influence of system buoyancy and hydraulic diameter.

Computational fluid dynamics analysis of buoyancy-aided turbulent mixed convection inside a heated vertical rectangular duct

Heat transfer in buoyancy-aided turbulent mixed convection inside a vertical rectangular duct with air was investigated through steady-state computational fluid dynamics (CFD) analysis with Reynolds-averaged Navier–Stokes turbulence models. Heat transfer can occur in the passive safety system of a nuclear reactor, which uses natural circulation as an initial driving force owing to its relatively low flow rate and strong heat flux condition. One example is the reactor cavity cooling system in a very high temperature gas-cooled nuclear reactor, whose high operating temperature enables high-efficiency power generation and hydrogen production. In buoyancy-aided turbulent mixed convection, heat transfer can deteriorate under specific conditions. Therefore, heat transfer must be analyzed because its deterioration can affect the performance of the safety system, which has an influence on the safety of the nuclear reactor. In this study, the results of a CFD analysis were compared with experimental results obtained from the riser heat transfer experimental facility, which is an experimental heat transfer facility with a 4.0 vertical rectangular duct test section. The CFD analyses were performed using a realizable k–ε model and a v2–f model, and the results were compared with the experimental results. It was confirmed that the results of the v2–f model exhibited good agreement with the experimental results. Based on this fact, four additional idealized cases were calculated using the v2–f model to investigate heat transfer deterioration in the turbulent mixed-convection regime. The calculation results revealed that the gradient of the velocity profile from the wall toward the center was reduced outside the near-wall region owing to buoyancy, which led to a reduction in the shear stress, turbulence generation, and heat transfer. In addition, the velocity gradient beyond the near-wall region reached a negative value above a certain buoyancy number, and turbulence generation and heat transfer started to recover. This analysis of heat transfer deterioration can provide insight into the construction of a heat-transfer coefficient correlation for buoyancy-aided turbulent mixed convection, and it will contribute to the development of heat transfer predictions and the evaluation of safety systems in nuclear reactors.

Effect of vent area on vented deflagration of hydrogen–methane–air mixtures

In this study, experiments on the vented deflagration of hydrogen–methane–air mixtures were performed in a vertical rectangular duct with a vent at its top end. A nondimensional vent coefficient (Kv) was employed to characterize the effects of vent area (Av) on flame evolution and pressure buildup within and outside the duct. The maximum internal overpressure was attained near the duct bottom for a certain Kv. As Kv was increased from 2.2 to 11.9, the maximum internal overpressure near the vent and at the center of the duct increased monotonically; however, a nonmonotonic trend was observed at the duct bottom. Helmholtz oscillations of the internal flame and overpressure were observed in the tests for low Kv, and acoustic oscillations of the internal overpressure with a frequency of ~1200 Hz appeared in all tests. The maximum external overpressure first increased and then decreased with an increase in Kv. The external explosion induced a pressure peak in the duct, and the effect of external explosion on the venting of the internal deflagrations was weakened as Kv ≥ 7.8.

EFFECTS OF VARIABLE THERMAL CONDUCTIVITY OF WATER-BASED NANOFLUIDS SATURATED WITH POROUS MEDIUM ON NATURAL CONVECTION HEAT TRANSFER ENHANCEMENT

EFFECTS OF VARIABLE THERMAL CONDUCTIVITY OF WATER-BASED NANOFLUIDS SATURATED WITH POROUS MEDIUM ON NATURAL CONVECTION HEAT TRANSFER ENHANCEMENT

Convective fluid flow and heat transfer in a vertical rectangular duct containing a horizontal porous medium and fluid layer

PurposeThe purpose of this paper is to investigate thermally and hydrodynamically fully developed convection in a duct of rectangular cross-section containing a porous medium and fluid layer.Design/methodology/approachThe Darcy–Brinkman–Forchheimer flow model is adopted. A finite difference method of second-order accuracy with the Southwell-over-relaxation method is deployed to solve the non-dimensional momentum and energy conservation equations under physically robust boundary conditions.FindingsIt is found that the presence of porous structure and different immiscible fluids exert a significant impact on controlling the flow. Graphical results for the influence of the governing parameters i.e. Grashof number, Darcy number, porous media inertia parameter, Brinkman number and ratios of viscosities, thermal expansion and thermal conductivity parameters on the velocity and temperature fields are presented. The volumetric flow rate, skin friction and rate of heat transfer at the left and right walls of the duct are also provided in tabular form. The numerical solutions obtained are validated with the published study and excellent agreement is attained.Originality/valueTo the author’s best knowledge this study original in developing the numerical code using FORTRAN to assess the fluid properties for immiscible fluids. The study is relevant to geothermal energy systems, thermal insulation systems, resin flow modeling for liquid composite molding processes and hybrid solar collectors.

Experimental study of atmospheric water collection powered by solar energy using the Peltier effect

A small-scale prototype with one thermoelectric module was constructed to collect water from the atmosphere, and studied experimentally. It was driven by a solar panel module and operated by reducing the cold side temperature to less than the dew point temperature of ambient air. The system was designed and assembled depending on the capacity of the Peltier device, of dimension (4×4 cm^2), and energy needed. The system consisted of a heat sink, extended cold surface, Peltier device and fan, which were housed in a vertical rectangular section duct and used with different air temperatures, airflow rates, and humidity levels. A numerical model was used to study the temperature distribution on the cold side, which was applied to size the cooler to estimate the water production rate. The results suggest that the water production rate increased with the increasing of the moisture level in the air. The tests also showed that increasing the amount of air flowing on the heat sink increased the amount of water collected and enhanced the transfer of heat on the hot side. The achieved results show that the collection of water is reasonable with the proposed thermoelectric method using solar energy.

Development of a heat transfer coefficient correlation for buoyancy-aided turbulent mixed convection of air inside a vertical channel

Passive cooling systems in many nuclear power plants utilize natural air circulation for cooling. When this cooling process occurs in a vertical duct, it concurrently has the characteristics of natural convection and forced convection; this state is referred to as mixed convection. The heat transfer rate for mixed convection of buoyancy-aided turbulent flow is less than that for forced convection at a corresponding Reynolds number. Previous studies on the mixed convection have focused on supercritical fluids; however, studies on heat transfer correlations for atmospheric air are limited. Therefore, in this study, experiments were conducted on the atmospheric air inside a vertical rectangular duct under the turbulent mixed convection condition, to determine the heat transfer correlations. The existing correlations for turbulent mixed convection were evaluated based on the experimental results. In addition, a new correlation was developed that yields the smallest discrepancy with the experimental results. The proposed correlation includes a modified buoyancy number and considers changes to the flow structure, according to the traveling distance of the flow. Finally, the proposed correlation was validated using existing data from experiments in which air was the working fluid.

Heat transfer in a vertical rectangular duct filled with a porous matrix and saturated with a nanofluid for temperature dependent viscosity

This paper presents the results of a comprehensive numerical study to analyze free convective heat transfer in a vertical rectangular duct filled with porous matrix and saturated with nanofluid for temperature dependent viscosity. Using the Darcy- Forchhiemer model, the momentum in the porous medium was simulated. While insulating two opposing wall ducts, the temperatures remained varied for the other two walls. Solutions to the transport equations for a Newtonian fluid were providednumerically through the finite difference approach to second order accuracy. The impact of the overriding parameters, for instance, the parameters of variable viscosity as well as inertial, the Grashof number, Darcy number, Brinkman number, solid volume index as well as the aspect ratio having a water-Copper nanofluid on the flow characteristics of natural convection is investigated. The flow nature using different nanoparticles is also studied. The values of volumetric flow rate, skin friction and Nusselt number are tabulated for pertinent parameters. It is found that for negative values of viscosity variation parameter the flow is enhanced in the lower part of the duct and positive values will enhance the flow in the upper part of the duct. Silver nanoparticle attains the maximum heat transfer rate when compared to other nanoparticles.Keywords: Natural convection; Darcy-Forchhiemer model; Nanofluids; Variable viscosity; Viscous dissipation; Rectangular duct; Finite difference method.

Buoyancy effects in vertical rectangular duct with coplanar magnetic field and single sided heat load – Downward and upward flow

The effects specific for downward and upward flows of liquid metal (LM) in a vertical rectangular duct in a coplanar magnetic field (MF) are investigated. The experiments were performed in the JIHT's test facility with a closed magnetohydrodynamic (MHD) mercury loop. The temperature and velocity fields were measured in the duct with single-side heating in a coplanar magnetic field. The averaged temperature fields, wall temperature distributions, statistical characteristics of temperature fluctuation and longitudinal velocity profiles were obtained. The results for the downward and the upward flow under different flow conditions are compared.

INFLUENCE OF TEMPERATURE-DEPENDENT CONDUCTIVITY ON CONVECTIVE HEAT TRANSFER IN A VERTICAL DUCT

An analysis has been carried out to study the flow and heat characteristics of a Newtonian fluid in a vertical rectangular duct. One of the vertical walls of the duct is cooled to a constant temperature, while the other wall is heated to constant but different temperature. The thermal conductivity is assumed to vary as a linear function of temperature. The basic equations governing the flow and heat transfer are highly non-linear coupled partial differential equations. Numerical solution of the problem is obtained by using finite difference method of second-order accuracy. The effects of various physical parameters such as conductivity parameter BK (-1 ≤ BK ≤ 1.0), Grashof number Gr (1.0 ≤ Gr ≤ 25.0), Brinkman number Br (0.01 ≤ Br ≤ 2.0), and aspect ratio A(0.5 ≤ A ≤ 2.0), which determine the velocity and temperature contours, are shown pictorially. Results are also presented for the skin friction, volumetric flow rate, and heat transfer rate for representative values of different key parameters. It is found that the intensity of the velocity contours is dense in the lower half region of the duct for negative values of conductivity variation parameter and in the upper region of the duct for positive values of conductivity variation parameter. The 3D contours on temperature are concave for negative values of BK and convex for positive values of BK.

Convective Heat Transfer in a Vertical Rectangular Duct aw- ith Temper ture Dependent Viscosity and Temperature-Dependent Thermal Conductivity

Convective Heat Transfer in a Vertical Rectangular Duct aw- ith Temper ture Dependent Viscosity and Temperature-Dependent Thermal Conductivity

Buoyancy effects in vertical rectangular duct with coplanar magnetic field and single sided heat load

In some DEMO blanket designs liquid metal flows in vertical ducts of rectangular cross-section between ceramic breeder units providing their cooling. Heat exchange in these conditions is governed by the influence of magnetic field (coplanar) and by buoyancy effects that depend on the flow orientation to the gravity vector (downward and upward flow). Magnetohydrodynamic and heat transfer of liquid metal in vertical rectangular ducts is not well researched. Experimental study of buoyancy effects in rectangular duct with coplanar magnetic field for one-sided heat load and downward and upward flowsis presented in this paper. The detail research with has been done on mercury MHD close loop with using of the probe technique allow to discover several advantageous and disadvantageous effects. The intensive impact of buoyancy force has been observed in a few regime of downward flow which has been laminarized by magnetic field. Due to the development in the flow of the secondary large-scale vortices heat transfer improved and the temperature fluctuations of the abnormally high intensity have been fixed. On the contrary, in the upward flow the buoyancy force stabilized the flow which lead to decreasing of the turbulence heat transfer ratio and, consequently, deterioration of heat transfer.

Heat Transfer in a Vertical Rectangular Duct Filled with a Nanofluid for Variable Viscosity

Heat Transfer in a Vertical Rectangular Duct Filled with a Nanofluid for Variable Viscosity

Numerical analysis of natural convective flow and heat transfer of nanofluids in a vertical rectangular duct using Darcy-Forchheimer-Brinkman model

In this paper, natural convective flow and heat transfer of nanofluids in a vertical rectangular duct filled with porous matrix is investigated. The Darcy-Forchheimer-Brinkman model is used to represent the fluid transport within the porous medium covering the parametric ranges of 1 ≤ Gr ≤ 25,0 ≤ Br ≤ 8, and 0.0001 ≤ Da≤ 100. Also, pure water and five different types of nanofluids (Cu, diamond, TiO2, Ag and SiO2) are used with a volume fraction range of 0% ≤ Ø ≤0.2%. The governing nonlinear, coupled partial differential equations for the two-dimensional laminar, steady flow and heat transfer are solved numerically by a finite difference method with second order accuracy. It is found that the heat transfer is enhanced due to the use of a nanofluid. Further, it is noticed that an increase in the Darcy or Grashof or Brinkman numbers, or the aspect ratio parameter increase the flow and heat transfer characteristics; whereas the inertial or the viscosity ratio parameters reduce the flow and heat transfer characteristics. It is observed from 2-D graphs that the fluid rise up from the middle portion of the vertical wall and flow down along the two horizontal walls forming symmetric rolls with clockwise and counter-clockwise rotation inside the cavity. The temperature contours in 2-D are smooth curves which span the entire enclosure, and they are generally symmetry with respect to the horizontal symmetric line. The results obtained reveal many interesting behaviors that warrant further study on the heat transfer enhancement due to the nanofluids.

Investigation of mixed convection and particle dispersion of nanofluids in a vertical duct

In the present paper, mixed convection of TiO2-water and CuO-water nanofluids in a laminar flow within a vertical rectangular duct is investigated numerically. The two-phase Euler-Lagrange approach is applied to simulate nanoparticles dispersion in the base fluid. Effects of nanoparticles concentration, aspect ratio, buoyancy and Brownian and Thermophoretic forces in a wide range of Richardson number ([Formula: see text]) on the hydrodynamics and thermal parameters are presented and discussed. It is observed that at [Formula: see text], dispersion of nanoparticles in the base fluid improves heat transfer rate more considerably. Whilst an improvement in convective heat transfer of CuO-water nanofluids is shown to be more than 22% at [Formula: see text], it does not exceed 15% at [Formula: see text]. Moreover, at [Formula: see text], particles disperse in the centre region of duct cross section and variation in aspect ratio does not alter the amount of enhancement of heat transfer significantly.

Buoyancy effects in vertical rectangular duct with coplanar magnetic field and single sided heat load

This article investigates an effect which was found out in downward flow of liquid metal (LM) in vertical rectangular duct in coplanar magnetic field (MF).The experiments have been performed on facility which located in JIHT. This facility is magneto hydrodynamic (MHD) mercury close-loop. The temperature field measurements have been performed at one side heating conditions in coplanar magnetic field. The averaged temperature fields, wall temperature distributions and statistical characteristics of temperature fluctuation have been obtained.The strong influence of counter thermo-gravitational convection (TGC) on average and fluctuation parameters has been observed. The influence of TGC in magnetic field leads to developing of temperature low-frequency fluctuations with high magnitude. The temperature fluctuation amplitude in a wide range of operating conditions is higher than turbulence level.