Numerical Investigation on the Performance of Two-Throat Nozzle Ejectors with Different Mixing Chamber Structural Parameters

In this study, the effects of the mixing chamber diameter (Dm), mixing chamber length (Lm) and pre-mixing chamber converging angle (θpm) were numerically investigated for a two-throat nozzle ejector to be utilized in a CO2 refrigeration cycle. The developed simulated method was validated by actual experimental data of a CO2 ejector in heat pump water heater system from the published literature. The main results revealed that the two-throat nozzle ejectors can obtain better performance with Dm in the range of 8–9 mm, Lm in the range of 64–82 mm and θpm at approximately 60°, respectively. Deviation from its optimal value could lead to a poor operational performance. Therefore, the mixing chamber structural parameters should be designed at the scope around its optimal value to guarantee the two-throat nozzle ejector performance. The following research can be developed around the two-throat nozzle geometries to strengthen the ejector performance.

The flow and heat performance of tree-like network heat sink with diverging–converging channel

A tree-like network heat sink with diverging–converging channel is designed, and effect of flow rate, channel diverging-converging angles on the flow and heat dissipation performance of the tree-like network heat sink is analysed and compared by numerical simulation. Results show that the diverging– converging angle of 2° can reduce the pressure drop by 14% when inlet mass flow rate is 0.00499kg/s. And the maximum temperature, the temperature difference between the maximum and minimum of the heat sink increases by 0.63K and 0.92K respectively. As the diverging-converging angle increases to 4°, however, it only reduces the pressure drop by 13% and can not bring more pressure drop due to formation of flow recirculation inside the tree-like network heat sink channel. Therefore, the diverging–converging fractal micro-channel heat sink with 2° has good heat dissipation performance with obvious lower pumping power.

Implication of geometrical configuration on heat transfer enhancement in converging minichannel using nanofluid by two phase mixture model: A numerical analysis

In the present study laminar forced convective flow of nanofluid through a converging minichannel is investigated numerically by employing two phase mixture model. The heat transfer enhancement and the corresponding pressure drop are analyzed for the following range of parameters: Reynolds number (700 ≤ Re ≤ 1650), particle volume concentration (0% ≤ ϕ ≤ 4%) and converging angle (θ = 0.029°, 0.043° and 0.05°). The results indicate that there is a considerable increase in pressure drop coupled with enhancement in heat transfer rate with particle loading due to the improvement in the thermal properties of the resulting mixture. The pressure drop in the converging channel increases with the converging angle. The pressure drop augments as high as 2 times by advancing the particle loading from 0% to 4%. The wall temperature decreases appreciably by 34 K and heat transfer coefficient is enhanced by as high as 98% from Re = 700, ϕ = 0% and straight channel to Re =1650, Hout = 2.75mm and ϕ = 4%. The enhancement in heat transfer and corresponding increase in pressure drop as compared to equivalent straight channel is presented by the performance factor, which increases with decrease in converging angle. There is a significant concern of the pumping power with increase in converging angle, volume fraction and Reynolds number.

Numerical study of mixed convection in a horizontal no parallel-plates channel with an unheated moving plate

Purpose The purpose of this paper is to analyze the thermal and fluid dynamic behaviors of mixed convection in air because of the interaction between a buoyancy flow and a moving plate induced flow in a horizontal no parallel-plates channel to investigate the effects of the minimum channel spacing, wall heat flux, moving plate velocity and converging angle. Design/methodology/approach The horizontal channel is made up of an upper inclined plate heated at uniform wall heat flux and a lower adiabatic moving surface (belt). The belt moves from the minimum channel spacing section to the maximum channel spacing section at a constant velocity so that its effect interferes with the buoyancy effect. The numerical analysis is accomplished by means of the finite volume method, using the commercial code Fluent. Findings Results in terms of heated upper plate and moving lower plate temperatures and stream function fields are presented. The paper underlines the thermal and fluid dynamic differences when natural convection or mixed convection takes place, varying minimum channel spacing, wall heat flux, moving plate velocity and converging angle. Research limitations/implications The hypotheses on which the present analysis is based are two-dimensional, laminar and steady state flow and constant thermo physical properties with the Boussinesq approximation. The minimum distance between the upper heated plate of the channel and its lower adiabatic moving plate is 10 and 20 mm. The moving plate velocity varies in the range 0-1 m/s; the belt moves from the right reservoir to the left one. Three values of the uniform wall heat flux are considered, 30, 60 and 120 W/m2, whereas the inclination angle of the upper plate θ is 2° and 10°. Practical implications Mixed convection because of moving surfaces in channels is present in many industrial applications; examples of processes include continuous casting, extrusion of plastics and other polymeric materials, bonding, annealing and tempering, cooling and/or drying of paper and textiles, chemical catalytic reactors, nuclear waste repositories, petroleum reservoirs, composite materials manufacturing and many others. The investigated configuration is used in applications such as re-heating of billets in furnaces for hot rolling process, continuous extrusion of materials and chemical vapor deposition, and it could also be used in thermal control of electronic systems. Originality/value This paper evaluates the thermal and velocity fields to detect the maximum temperature location and the presence of fluid recirculation. The paper is useful to thermal designers.

Heat transfer and friction factor in a dimple-pin fin wedge duct with various dimple depth and converging angle

Purpose – The dimple is adopted into a pin fin wedge duct which is widely used in modern gas turbine vane cooling structure trailing edge region. The purpose of this paper is to study the effects of dimple depth and duct converging angle on the endwall heat transfer and friction factor in this pin fin wedge duct. Design/methodology/approach – The study is carried out by using the numerical simulations. The diameter of dimples is the same as the pin fin diameter with an inline manner arrangement in relation to the pin fin. The ratio between dimple depth and dimple diameter is varied from 0 to 0.3 and the converging angle is ranging from 0° to 12.7°. The Reynolds number is between 10,000 and 50,000. Results of the endwall Nusselt number, friction factor, and flow structures are included. For convenience of comparison, the pin fin wedge duct with a converging angle of 12.7° without dimples is considered as the baseline. Findings – It is found that the dimples can effectively enhance the endwall heat transfer due to the impingement on the dimple surface, reattachment downstream the dimple and recirculation in front of the pin fin leading edge. By increasing the converging angle, the heat transfer is also increased but with a large friction factor penalty. In addition, the heat transfer enhancement for deep depth cases is 1.57 times higher than that of the low depth case. The thermal performance indicates that the intensity of heat transfer enhancement depends upon the dimple depth and converging angle. Originality/value – It suggests that the endwall heat transfer in a pin fin wedge duct can be increase by the adoption of dimples. The optimal dimple relative depth is 0.2 with low friction factor and high heat transfer performance.

Effect of Inlet Nozzle Shape on Performance of Darrieus-Type Hydro-Turbine Operated in Small Open Water Channel

A Darrieus-type hydro-turbine has been developed for the utilization of extra low head hydraulic energy. In the case of a ducted Darrieus-type hydro-turbine which consists of an intake, a runner, a casing and a draft tube, it has been found that the Darrieus runner with the narrow intake can generate larger torque without deterioration of efficiency than that with the parallel intake with the constant width. In this paper, the effect of the shape of the inlet nozzle on the performance of the Darrieus-type hydro-turbine operated in open channel flow is investigated both experimentally and numerically. Tested nozzles are two types of two-dimensional symmetric inlet nozzle, Half Diameter curved nozzle (HD nozzle) and Straight Line nozzle (SL nozzle). As a result, the Darrieus hydroturbine with SL nozzle generates larger power and yields higher efficiency than that with HD nozzle. In addition, the effects of nozzle converging angle and outlet width of SL nozzle on the turbine performance are investigated. As a result, it is found that the Darrieus hydroturbine with SL nozzle having large converging angle generates larger power with higher efficiency than that with the nozzle having small converging angle. And then, it is found that the generated power increases when SL nozzle with large outlet width is installed.

Effect of Channel Spacing on Mixed Convection in Vertical Convergent Channels

Mixed convection in air in a convergent channel with the two principal flat plates at uniform heat flux is analyzed numerically by Fluent code. In the considered system two parallel adiabatic extensions are placed downstream of the convergent channel. The forced flow is obtained by imposing a pressure drop between the inlet and the outlet of the channel. The flow in the channel is assumed to be two-dimensional, turbulent and incompressible. A k-ε turbulent model is employed. Results in terms of dimensionless wall temperature distribution as a function of the walls converging angle, the Grashof number, the pressure drop and the channel aspect ratio are presented in the ranges: 0° ≤ θ ≤ 10°; 4.10 102 ≤ Gr ≤ 32.1 105, 0 ≤ ΔP ≤ 8.82·107, 10.15 < Lw/bmin < 58.0. Results show that Reynolds number, and then the mass flow rate flowing in the channel, increases at decreasing aspect ratios, Lw/bmin. The converging angle that optimizes the fluid-dynamic within the channel is equal to 5°. Dimensionless maximum wall temperature values decreases at increasing Reynolds number and the larger the aspect ratio, the larger the decrease. The Reynolds number over which natural convection become negligible, with respect to forced convection, increases at increasing converging angle and at decreasing aspect ratio.

Mixed Convection Heat Transfer in a Convergent Vertical Channel With a Moving Plate

A numerical investigation of mixed convection in air in a convergent vertical channel, due to the interaction between a buoyancy flow and a moving plate induced flow, is presented. The plate moves at a constant velocity along the buoyancy force direction and the principal inclined walls of the channel are heated at uniform heat flux. The numerical analysis is carried out by means of the finite volume method, using the commercial code Fluent. The effects of the channel spacing, wall heat flux, moving plate velocity and converging angle are investigated. Heated wall temperature increases at increasing converging angle, except for natural convection in a 10 mm minimum channel gap. The effect of the converging angle on the wall temperatures is less marked at the larger channel spacing. Maximum temperature of the moving plate is attained in the parallel wall channel for a 30 W m−2 wall heat flux, both in the 10 mm and 40 mm channel, whereas for a 220 W m−2 wall heat flux in the 40 mm channel in mixed convection, maximum wall temperatures are exhibited for a 10° angle. Nusselt, Reynolds and Richardson numbers are correlated by a monomial equation for each converging angle and a unique monomial correlation for all investigated angles in the 2.1·10−2 – 5.1·105 Richardson number range is presented.

Analysis of Mixed Convection in Vertical Convergent Channels

Air mixed convection in a convergent channel with the two principal flat plates at uniform heat flux is analyzed numerically. In the considered system two parallel adiabatic extensions are placed downstream the convergent channel. The forced flow is obtained by imposing a pressure drop between the inlet and the outlet of the channel. The flow in the channel is assumed to be two-dimensional, turbulent and incompressible. A k-ε turbulent model is employed. Results in terms of dimensionless wall temperature distribution as a function of the walls converging angle, the Grashof number and the pressure drop are presented in the ranges: 0 ≤ ΔP ≤ 2.2·107, 2.8·104 ≤ Gr ≤ 2.1·105. Results show that increasing the angle of converging the Reynolds number increases at the same pressure drop. The larger the pressure drop the smaller the contribution of the free convection to the Reynolds number. Increasing the converging angle only slightly increases the ΔP value for which the effect of free convection is negligible.