Experimental and computational analysis of thermal mixing characteristics of a coaxial jet

https://doi.org/10.1016/j.expthermflusci.2016.11.028Get rights and content

Highlights

  • Experimental and numerical analyses of thermal mixing of a coaxial jet.

  • The dominant frequency of temperature fluctuation is found as 5 Hz.

  • There is a good agreement between experimental and numerical studies with respect to spectral analyses.

  • Thermal mixing efficiency increases with increasing temperature difference between hot and cold jet.

Abstract

In this study, experimental and numerical analyses have been performed to investigate the mixing behavior of hot and cold fluids for a coaxial jet. Thermal mixing phenomenon is an important issue for many industrial applications, since thermal stress occurs when fluids do not mix completely. Several test cases were considered to investigate the mixing quality and frequency of temperature fluctuations. Computational study was performed using Large Eddy Simulation (LES) turbulence model, since this model is proven in this type calculations. A commercial CFD code, ANSYS-Fluent is used for numerical calculations. The study is performed for governing parameters as ratio of mass flow rate of hot (annular) jet to cold (central) jet and temperature difference between hot and cold jet. It is found that there is a good agreement between experimental and numerical studies with respect to spectral analyses. The dominant frequency of temperature fluctuation is found as 5 Hz as compatible with literature. Thermal mixing efficiency increases with increasing temperature difference between hot and cold jet. Also, thermal mixing performance getting better with enhancing flow rate of hot jet in the first half of the channel and the best mixing observed at ṁh/ṁc = 2 along second half of the channel.

Introduction

Thermal mixing of fluids at various temperatures is commonly encountered in many fields of industry and engineering applications such as nuclear, mechanical and automotive engineering. In these kinds of applications, temperature fluctuations occur in the mixing region of cold and hot fluid. These fluctuations may cause thermal stress and subsequently cracks in surrounding surfaces of the fluid. For instance, in 1992, extensive cracking was found in a control rod guide tube that had been removed from the core of the UK Prototype Fast Reactor (PFR). High-cycle thermal fatigue was found to be the cause of the cracks in the connecting pipe and the middle-stage heat exchange shell at the Tsuruga-2 PWR (Japan) in 1999 [1]. Such industrial cases revealed that controlling of thermal mixing phenomena in industrial systems is an important subject. Using coaxial jets to increase mixing effectiveness of hot and cold fluid is an effective method because of geometrical structure of the jet.

Lu et al. [2], carried out an experimental study on the three-dimensional (3D) temperature fluctuation caused by the coaxial-jet flows based on the relatively integrated velocity vector field. The measurement made by particle image velocimetry (PIV) and the time variation of temperature measured by the thermocouples. Based on measured data, it was found that the time-averaged parameters were axial symmetric, while the transient temperature fluctuations were not. Also, transient temperature fluctuations in circumferential direction are asynchronous with similar distributions of the amplitude and frequency, while those in radial direction are synchronous with different amplitude and similar frequency distributions.

Mixing characteristics and the effects of inflow pulsation on the flow behaviors of turbulent confined coaxial jet flows have been studied numerically by Jang and Sung [3]. Large eddy simulations were performed at Re = 9000 and the mean velocity ratio of the central to annular jet was 0.6. The optimal phase difference conditions for mixing enhancement and the decrease in the reattachment length were obtained when the strength of the outer vortices was high. They found that the strength of the inner vortices was reduced by varying the phase difference, and the reattachment length was minimized, and that if the strength of the inner vortices was increased, mixing was enhanced.

Balarac et al. [4] performed direct numerical simulations (DNS) to investigate mixing in free round coaxial jets. Main objective of the study is to investigate the influence of upstream conditions upon the global flow structure and the mixing process. The mixing phenomena are studied through the spatial and temporal development of the mixture fraction of the annular and the inner fluids. The results showed that the turbulent mixing process and the mixture fraction field in coaxial jets depend on the conditions of upstream.

Wang and Mujumdar [5], studied the three-dimensional flow and mixing characteristics of multiple and multi-confined turbulent opposing jets in a pipe, numerically. Standard k-ε turbulence model was used as turbulence model. Based on obtained data it is concluded that multiple opposing jets can achieve better mixing than single opposing jets.

Wang et al. [6], made a numerical investigation to study the mixing performance of opposing jets in a confined channel. Some new geometric conditions were performed to increase effectiveness of mixing. They observed that dissimilar inlet momenta and unequal slot width could significantly improve the mixing performance; this improvement depended strongly on the operating conditions and geometric configurations. Addition of baffles in the exit of the channel increases the mixing quality. The pressure loss was found to depend strongly on the mixer geometry and operating conditions.

Chandran et al. [7], carried out a numerical study to investigate thermal striping in Prototype Fast Breeder Reactor (PFBR). Ten-jet water model which consist from seven hot and three cold jets was used in the study. The cold jet Reynolds number was taking constant while hot one has been varied. Based on the results, the maximum temperature fluctuation was seen when hot and cold jet velocities are same and the lattice plate contribute higher thermal striping performance than core cover plate. Durve et al. [8] made a numerical study to analyses flow behaviors of single, two parallel and three parallel jet flows. Effects of jet spacing, merge - combine point of jets and velocity ratio on flow field were working parameters of the study. It was found from the calculated data that the merge point of the jet flow is affected from spacing between the jets and the jet outlet. Also velocity ratio of jets drastically changes location of merge and combining point of the jets. Naik-Nimbalkar et al. [9] performed an experimental and numerical investigation of thermal mixing phenomena of single/twin jets in cross flow. Based on their findings, prediction of mean temperature, magnitude of the temperature fluctuations, characteristic frequencies of temperature fluctuations, attenuation of the temperature fluctuations in the boundary layer near the pipe wall, effect of multiple jets entering in the cross flow and effect of space between twin jets was investigated. Consequently, it was found that numerical predictions are in good agreement with experimental measurements.

Jung and Yoo [10] carried out Large Eddy Simulations to investigate the effect of inlet thermal intensities (Trms/T) on thermal mixing of the triple jet flow. Smagorinsky–Lilly and the k–l sub-grid scale models were used in the turbulence model. The calculations were confirmed by comparing with the available experimental data. It is found that LES predict faster decay of mean temperature along the axis of the central jet. The inlet values of thermal intensity and the sub-grid scale models had no effect on the solution.

Kok et al. [11] made a numerical investigation in a body inserted channel to analyze fluid flow and thermal mixing behavior of two parallel jets which are in different temperatures. Bodies in different aspect ratios were located for different cases into the channel with the aim of increase thermal mixing efficiency inside the channel. It is found that mixing performance increases in the first half of the channel (x/L = 0–0.5) with increasing of Re number and chosen parameters can be used as control mechanism of thermal mixing in the channel. Suyambazhahan et al. [12], perform a numerical study of flow and thermal oscillation phenomena of twin jets. Effects of jet nozzle spacing, jet inlet temperature and jet width are investigated in the study. Based on calculated data it was found that even in the turbulent forced convection regime, buoyancy has important effect on the jet flow oscillation behaviors. It also has significant influence on recirculation zones and merging point of jets. Varol et al. [13] investigated the thermal mixing phenomena of parallel twin jet in a square body inserted inclined narrow channel, experimentally. Flow ratio of hot jet to cold jet, inclination angle of the channel, temperature difference between jets and adiabatic square body were used as experimental parameters. It is found that inclination angle of the channel plays important role on the thermal mixing performance. Also, other chosen parameters can be used to control thermal mixing.

An experimental investigation was carried out by Kok et al. [14] to analyze the thermal mixing phenomena in a rectangular cross-section narrow channel. Inclination angle of the channel, ratio of hot jet to cold jet, temperature difference between hot and cold jets and jet inlets diameters were used to control thermal mixing behaviors. In addition, an artificial neural network (ANN) model was built with limited number of experimental measurements for a forward model. Results showed that the inclination angle of the channel has considerable effect on fluid flow and thermal mixing efficiency. The obtained results from ANN model indicated that estimations can predict the output parameters without carry out any experiment.

Kok et al. [15] made an experimental study to investigate thermal mixing phenomena of parallel twin jets in circle shaped passive obstacle inserted narrow confined channel. Parallel jets have different temperatures and water is working fluid. The channel can be inclined from 0° to 90°. The vertical position of the channel is 90°. Also temperature difference values and flow rate of hot jet to cold jet are subaltern parameters. Results demonstrate that better thermal mixing is achieved in vertical position of the channel and circle shaped obstacle increase mixing performance of the fluid.

Ayhan and Sökmen [16] performed a Computational Fluid Dynamics (CFD) study of turbulent thermal mixing in a T-junction and compared simulation results with the experimental results. The main purpose of this study is to get an idea about the magnitude and frequency of temperature fluctuations for thermal striping phenomenon by analyzing power spectra of temperature fluctuations. In this study, Large Eddy Simulation (LES) computations with an eddy-viscosity type subgrid-scale stresses model resulted in sufficiently accurate predictions of temperature and velocity fluctuations. It is stated that these fluctuations are important in order to characterize thermal fatigue. Also it is reported that the results of Reynolds Averaged Navier-Stokes (RANS) computations, steady or unsteady, failed to provide accurate results.

Ayhan and Sökmen [17] performed a CFD study of turbulent thermal mixing of two water streams having different temperatures in a T-junction for three different cases. Mass flow rate of main and branch duct inlet is constant for all cases however; momentum ratios are different. Three different branch pipe diameter were considered in this study. Temperature and velocity fluctuations, which are important for characterizing thermal mixing, are observed by using LES turbulence model. Temperature and velocity data give information about magnitude and intensity of thermal load and the length required to reach the thermal and hydraulic equilibrium in related study. Analysis of the temperature fluctuations shows that the frequency range of 2–5 Hz contains most of the energy, therefore, may cause fatigue.

The main aim of this work is to investigate the experimental and numerical analysis of co-axial jets. LES turbulence model is used for numerical analysis. The literature survey reveals that most of thermal mixing problems are focused on flow and mixing phenomena of parallel or perpendicular jets. There are few articles on flow field and thermal mixing characteristics of coaxial jets. Using coaxial jets for mixing of fluids is effective method because of geometrical advantages. In this study a coaxial jet nozzle designed and inserted into a confined channel.

Section snippets

Experimental set-up

The experimental set-up mainly consist from test channel, hot/cold water tanks, heat exchanger, pumps, flow meters, control panel, computer and data logger as demonstrated in Fig 1. The working diagram of the experimental set-up is also given in Fig. 2. As seen from the figure, the mixed flow leaves the test channel from one hole. The mixed flow is divided into two branches which one turn back to cold tank and the other to hot tank. The cold branch fluid temperature is decreased to room

Computational domain and mesh structure

Computational part of the experimental setup is shown in Fig. 6. This is a square duct having dimensions of 12Dx12Dx50D as in experimental setup. Boundary conditions are also shown in the figure and also tabulated in Table 1. For all cases both cold and hot inlet regimes are turbulent except hot inlets at cases 1 and 2 (these are having transition regime).

ANSYS Meshing 15.0 is used to create mesh structure of computational domain. Detailed mesh structure is presented in Fig. 7. Hexahedral mesh

Numerical method

Fluid behavior in flow is characterized by Navier-Stokes equations. In LES model as an unsteady turbulence approach, filtered Navier-Stokes equations are used to solve turbulence. Separation of large scales (filtered components) and small scales (sub filtered components) is made by using filtering operation. In LES model, the small-scale eddies are modeled (since behaviors of small scale eddies are unique) using sub grid scale models and large scale eddies are solved directly by using filtered

Definition of mixing index

During mixing processes of fluids that have different temperatures, better thermal mixing of hot and cold fluid is expected to be achieved. To measure the thermal mixing performance of such a mixing under various working conditions, a Mixing Index (MI) is recommended by Wang and Mujumdar [5]. This MI is used as a measure of the closeness of the temperature profile to the mean temperature at any axial location in the channel:MI=StΔT×100where St based on the average temperature, is the standard

Results and discussion

In this study six different experimental cases are simulated. In each case, temperature fields are recorded at several points. Numerical studies also performed for these six cases. Data collecting frequency is 100 Hz for both numerical and experimental studies. Boundary conditions for all cases are listed in Table 1. Some cases have same temperature differences between cold and hot fluids, some cases have same flow rate conditions and some cases have different conditions for detailed comparison.

Conclusions

An experimental and a CFD study of turbulent thermal mixing of two water streams having different temperatures in a coaxial jet were performed and compared each other. LES turbulence model with WALE SGS model is used to solve turbulence. Experimental procedure and numerical calculations were performed for six different boundary condition cases. Mixing characteristics and spectral behaviors were investigated for both experimental and numerical studies.

The comparison of MI behaviors gives us that

Acknowledgements

Authors thank to the Scientific and Technological Research Council of Turkey (TUBİTAK) for their valuable financial support with a project number 114M584.

References (18)

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