Implementation of Linear Stability Theory on Hollow Cone-shaped Liquid Sheet

Surface instability of a swirling liquid sheet emanating from a centrifugal injector in presence of external and internal gas flows is studied in this paper. A three-dimensional flow for the liquid sheet and two-dimensional flows for external and internal gas flows are considered. The set of equations involved in this analysis differs from the earlier analyzes. In previous studies, a cylindrical liquid sheet has been considered to implement the linear theory but in this study, the linear stability theory is implemented on a cone-shaped liquid sheet for different cone angles. Actually more over than axial and tangential movements, the radial movements of liquid sheet and gas flows are considered in the present model. Due to complexity of the derived governing equations, semi-analytical and numerical methods were applied to solve them. The case study is oxidizer injector of rocket engines. Implementation of linear stability theory on a hollow cone-shaped liquid sheet better can predict instability phenomenon than the general linear stability analysis for this type of liquid sheets. The results show very close agreement with available experimental data

Numerical Investigation of Nanofluid Forced Convection in Channels with Discrete Heat Sources

Numerical simulation is performed to investigate the laminar force convection of Al2O3/water nanofluid in a flow channel with discrete heat sources. The heat sources are placed on the bottom wall of channel which produce much thermal energy that must be evacuated from the system. The remaining surfaces of channel are kept adiabatic to exchange energy between nanofluid and heat sources. In the present study the effects of Reynolds number (Re=50,100,200,400, and 1000), particle volume fraction (=0 (distilled water), 1 and 4%) on the average heat transfer coefficient (h), pressure drop (Δ), and wall temperature () are evaluated. The use of nanofluid can produce an asymmetric velocity along the height of the channel. The results show a maximum value 38% increase in average heat transfer coefficient and 68% increase in pressure drop for all the considered cases when compared to basefluid (i.e., water). It is also observed that the wall temperature decreases remarkably as Re and ϕ increase. Finally, thermal-hydraulic performance (η) is evaluated and it is seen that best performance can be obtained for Re=1000 and =4%

Transport processes in laminar and turbulent opposing jet contactors

A computational study is carried out to investigate the heat, mass and momentum transfer phenomena in a novel two-dimensional opposing jet gas-particle contactor. Initially, the confined laminar opposing jet configuration is examined by solving the unsteady and steady conservation equations. Both equal and unequal momentum jets impinging normally against each other are considered. The thermal boundary conditions tested are: uniform wall temperature and insulated walls. Predicted velocity fields, pressure drop, and heat transfer coefficient distributions are compared with corresponding values for laminar flow between parallel plates. Mixing of the jets in such flows is discussed in terms of the variation in temperature profiles in the exit channel.An evaluation of the predictions of different high- and low-Reynolds number turbulence models for turbulent flow and heat transfer in confined impinging and opposing jet configurations is carried out. The heat transfer features of these systems are compared on the basis of equal mass flow rate in the two systems. A parametric study for flow and heat transfer in the turbulent opposing jet case is carried out.The motion and drying of single particles moving in two dimensional opposing jet contactor in the turbulent regime are investigated. The effects of key parameters such as particle size, release position in the inlet jet, jet Reynolds number, and geometry of the contactor on particle residence time and particle moisture content are examined, assuming a power-law model for the falling rate drying kinetics.A two-phase flow model is developed based on a modified stochastic particle-trajectory model. Both turbulence modulation and particle dispersion effects are incorporated in the code. The effect of turbulence intensity on particle-gas heat transfer is included via a new model. A modified algorithm is introduced for the solution of the governing equations.The proposed mathematical model and solution algorithm are applied to selected test problems, e.g. dilute gas-particle flow in a channel. The predicted results compared favorably with results of independent investigations.The two-phase code is employed for a parametric study of the flow and drying characteristics of wet particles in the proposed opposing jet contactor using superheated steam as the drying agent. Predictions are made for various degrees of superheat and operating pressure. The effects of particle size and jet Reynolds numbers are also investigated. The particle residence time and moisture content distributions are predicted and a new method is suggested for an effective use of computer simulation results in the design and analysis of convective dispersion dryers for particulate materials

Coriolis and buoyancy effects on heat transfer in viewpoint of field synergy principle and secondary flow intensity for maximization of internal cooling

The present investigation emphases on rotation effects on internal cooling of gas turbine blades both numerically and experimentally. The primary motivation behind this work is to investigate the possibility of heat transfer enhancement by dean vortices generated by Coriolis force and U-bend with developing turbulent in the view point of the field synergy principle and secondary flow intensity analysis. A two-passage internal cooling channel model with a 180° U-turn at the hub section is used in the analysis. The flow is radially outward at the first passage of the square channel and then it will be inward at the second passage. The study covers a Reynolds number (Re) of 10,000, Rotation number (Ro) in the range of 0–0.25, and Density Ratios (DR) at the inlet between 0.1–1.5. The numerical results are compared to experimental data from a rotating facility. Results obtained with the basic RANS SST k-ω model are assessed completely as well. A field synergy principle analysis is consistent with the numerical results too. The results state that the secondary flows due to rotation can considerably improve the synergy between the velocity and temperature gradients up to 20%, which is the most fundamental reason why the rotation can enhance the heat transfer. In addition, the Reynolds number and centrifugal buoyancy variations are found to have no remarkable impact on increasing the synergy angle. Moreover, vortices induced by Rotation number and amplified by Reynolds number increase considerable secondary flow intensity which is exactly in compliance with Nusselt number enhancement

A comprehensive analysis of heat transfer in a heat exchanger with simple and perforated twisted tapes based on numerical simulations

One of the most important methods to promote heat transfer (HT) in a heat exchanger (HE) is to use a twisted tape (TT) inside tubes. Therefore, in this paper, the result of the effect of TTs on the Nusselt number (Nu), and the performance coefficient of HEs in a laminar flow are investigated. plain tube (PT), simple TTs and perforated twisted tapes (PTTs) at 3 torsion ratio, blade numbers (N) 2, 4, 6 and 8 in the Reynolds numbers (Re) from 200 to 1000 are investigated. Ansys-Fluent computing software is used for this simulation. The findings indicate a positive correlation between the number of blades and both the Nu and the friction factor. It is important to acknowledge that the objective is to enhance the efficiency of the higher HE. Additionally, since there is an undesirable ratio between the rise of the Nu and the boost of the friction factor, so the HE coefficient decreases as the number of blades in TTs increases, from 6 to 8. The PTT with 6 blades provides the best performance coefficient. Compared to the simple TT, in the PTT, the Nu increases to 15.24%, the friction factor decreases to 22.26%, and the coefficient of thermal performance grows to 18.07%

Using Neural Network for Determination of Viscosity in Water-TiO Nanofluid

Using nanofluids is a novel solution to enhance heat transfer. This study tries to extract the model of viscosity changes in water-TiO 2 nanofluid through examining the effect of temperature and volume fraction on the viscosity. Results were recorded and analyzed within temperature range of 25 to 70°C with increments of five for 0.1, 0.4, 0.7, and 1% volume fractions. The obtained results demonstrated that the viscosity of this nanofluid decreases by increasing the temperature and increases by raising the volume fraction. The results show that conventional correlations are unable to properly predict nanofluid viscosity especially at high volume fractions. A model was developed by the data obtained from experiments to estimate viscosity of water-TiO 2 nanofluid based on two variables of temperature and volume fraction using neural network. The proposed model was qualified as highly competent for determination of nanofluid viscosity

Application of maximum entropy principle for estimation of droplet-size distribution using internal flow analysis of a swirl injector

The maximum entropy principle is one of the first methods, which have been used to predict droplet size and velocity distributions of liquid sprays. Due to some drawbacks in this model, the predicted results do not match well with the experimental data. This paper presents a different approach for improving the maximum entropy principle model. It is suggested to improve the available energy source in the maximum entropy principle model equation by numerical solution of flow inside the injector based on the computational fluid dynamics technique. This will enhance the calculation accuracy of the turbulent kinetic energy of the output spray. Application of this procedure enhances the model predictions. The liquid sheet properties resulted from the analysis are also applied for calculation of the momentum source in the maximum entropy principle model. The proposed model is applied to predict the droplet size distribution of a hollow-cone spray formed by a swirl injector. The results show a better agreement with the available experimental data than the results of prior models

Experimental Study on Chaotic Mixing Created by a New Type of Mixer with Rotational Blades

The aim of this paper is an experimental investigation of laminar mixing in a new type of chaotic mixer, which has been proposed by Hwu (2008), by means of material line deformation. The mixer is a circular cavity with two rotational blades which move along a semicircular path and drive the fluid motion. The flow visualization is carried out by marking of the free surface of the flow with a tracer in working fluid. In the present study the effects of length and rotational speed of blades on mixing efficiency are evaluated by measuring of the area covered by the tracer. As a result, it is demonstrated that the goodness of mixing increases as rotational speed of blades increases. Also, it is detected that the mixing efficiency strongly depends on the lengths of rotating blades