scholarly journals Experimental investigation of thermodynamic instability of supercritical endothermic hydrocarbon fuel within a small-scale channel

2019 ◽  
Vol 11 (3) ◽  
pp. 168781401983028
Author(s):  
Hui Wang ◽  
Wansheng Nie ◽  
Lingyu Su
Keyword(s):  
Heat Transfer ◽  
Kinetic Energy ◽  
Turbulent Flow ◽  
Temperature Drop ◽  
Hydrocarbon Fuel ◽  
Small Scale ◽  
Operating Pressure ◽  
Fuel Temperature ◽  
Transition Flow ◽  
Endothermic Hydrocarbon Fuel

A series of experiments were performed to investigate the thermodynamic instabilities that occur during heating of supercritical endothermic hydrocarbon fuel. A “power–temperature drop” characteristic curve is used to analyze the mechanism of thermodynamic instabilities. The results indicate that the heat-transfer process in a heated tube with increasing heating power can be divided into three periods: stable, developing, and instable; in which, the thermodynamic instabilities are found to occur. When the outlet fuel temperature reaches the pseudo-critical temperature, an acute decrease in fuel density and viscosity causes the flow to change from a transition flow to a turbulent flow, and the sharp increase of heat transfer in turbulent flow increases the thermodynamic instabilities. The intensity of the instability is related to the kinetic energy of the flow and the oscillatory extent. When the mass flow rate is increased from 1.0 to 1.5 g/s, the effect on the flow’s kinetic energy dominates the change in instability which causes the intensity of the instability to increase. While the intensity of the instability decreases with increasing inlet fuel temperature, which results from the decrease of the oscillatory extent. The effects of the operating pressure on the instability are not linear because of the properties of fuel change, obviously with pressure near the critical point.

Effect of Groove Dimension on Thermal Performance of Turbulent Fluid Flow in Internally Grooved Tube

2016 ◽  
Author(s):  
Sogol Pirbastami ◽  
Samir Moujaes
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Pressure Drop ◽  
Kinetic Energy ◽  
Turbulent Flow ◽  
Friction Factor ◽  
Turbulent Kinetic Energy ◽  
Thermal Performance ◽  
Tube Length ◽  
Pitch Length

A Computational Fluid Dynamics (CFD) study of heat enhancement in helically grooved tubes was carried out by using a 3-dimensional simulation with the STARCCM+ simulation package software. The k-ε model selected for turbulent flow simulation and the governing equations were solved by using the finite volume method. Geometric models of the current study include 3 rectangular grooved tubes with different groove width (w) and depth (e) which varies from 0.2 mm to 0.6 mm for the same tube length of 2.0m and diameter of 7.1 mm. The simulations were performed in the Reynolds number (Re) range of 4000–10000 with a uniform wall heat flux of 3150 w/m2 applied as a boundary condition on the surface of each tube. The purpose of this research is to investigate the effect of different groove dimensions on the thermal performance and pressure drop of water inside the grooved tubes and clarify the structural nature of the flow in regards to flow swirl and turbulent kinetic energy distributions. It was found that the highest performance belongs to the groove with these dimensions (w = 0.2 mm and e = 0.2 mm) which was considered for further study. Then, for these same groove dimensions four pitch size to tube diameter (p/D) ratios ranging from 1 to 18 were simulated for the same 2.0 m length tube. The results for Nusselt number (Nu) and friction factor (f) showed that by increasing the (p/D) ratio both the Nu numbers and the friction factors (f) values decrease. With a smaller pitch length (p) the turbulence intensity generated by the internal groove was also found to increase. The physical behavior of the turbulent flow and heat transfer characteristics were observed by contour plots which showed an increasing swirl flow and turbulent kinetic energy as p/D decreases. With an increase of the Nu number for smaller p/D ratio, a penalty of a higher pressure drop was obtained. The results were validated with a previous experimental work and the average error between the experimental and CFD Nu numbers and f were 13% and 8% respectively. A higher level of turbulent kinetic energy is observed near the grooves, as compared to the smooth areas of the pipe surface away from the grooves, which are expected to lead to higher levels of heat transfer. The effect of pitch length (p) on the flow pattern were plotted by streamlines along the tubes, by decreasing the pitch size (p/D ratio) an increase in the swirl is noticed as evidenced by the plots of the path lines. Finally, empirical correlations for Nusselt number and friction factor were provided as a function of p/D and Re number. This study indicates that the incorporation of the internal groove, of particular dimensions, can lead to an improvement of performance in heat exchanger devices. A limited variation of the groove dimensions was conducted and it was found that the values of Nu and f do not improve with an increase of (w) nor with that of (e) from 0.2–0.6 mm.

CFD Simulation of a Coolant Flow and a Heat Transfer in a Pebble Bed Reactor

2008 ◽  
Author(s):  
Wang-Kee In ◽  
Won-Jae Lee ◽  
Yassin A. Hassan
Keyword(s):  
Heat Transfer ◽  
Gas Flow ◽  
Cfd Simulation ◽  
Reverse Flow ◽  
Temperature Drop ◽  
Coolant Flow ◽  
Normal Operation ◽  
Fuel Temperature ◽  
Pebble Bed Reactor ◽  
Detailed Simulation

This CFD study is to simulate a coolant (gas) flow and heat transfer in a PBR core during a normal operation. This study used a pebble array with direct area contacts among the pebbles which is one of the pebbles arrangements for a detailed simulation of PBR core CFD studies. A CFD model is developed to more adequately represent the pebbles randomly stacked in the PBR core. The CFD predictions showed a large variation of the temperature on the pebble surface as well as in the pebble core. The temperature drop in the outer graphite layer is smaller than that in the pebble-core region. This is because the thermal conductivity of graphite is higher than the fuel (UO2 mixture) conductivity in the pebble core. Higher pebble surface temperature is predicted downstream of the pebble contact due to a reverse flow. Multiple vortices are predicted to occur downstream of the spherical pebbles due to a flow separation. The coolant flow structure and fuel temperature in the PBR core appears to largely depend on the in-core distribution of the pebbles.

Heat transfer and cracking performance of endothermic hydrocarbon fuel when it cools a high temperature channel

2016 ◽  
Vol 149 ◽  
pp. 112-120 ◽  
Author(s):  
Lei Yue ◽  
Jianzhou Wu ◽  
Yu Gong ◽  
Jingwei Hou ◽  
Liangping Xiong ◽  
...  
Keyword(s):  
Heat Transfer ◽  
High Temperature ◽  
Hydrocarbon Fuel ◽  
Cracking Performance ◽  
Endothermic Hydrocarbon Fuel

An Experimental Study of Chilton–Colburn Analogy Between Turbulent Flow and Convective Heat Transfer of Supercritical Kerosene

2017 ◽  
Vol 139 (6) ◽  
Author(s):  
Yongjiang Zhang ◽  
Fengquan Zhong ◽  
Yunfei Xing ◽  
Xinyu Zhang
Keyword(s):  
Heat Transfer ◽  
Friction Coefficient ◽  
Turbulent Flow ◽  
Convective Heat Transfer ◽  
Skin Friction ◽  
Convective Heat ◽  
Control Volume ◽  
Skin Friction Coefficient ◽  
Fuel Temperature ◽  
Number Range

In this paper, characteristics of turbulent flow and convective heat transfer of supercritical China RP-3 kerosene in a horizontal straight circular tube are studied experimentally, and the validity of Chilton–Colburn analogy is examined. Using a three-stage heating system, experiments are conducted at a fuel temperature range of 650–800 K, a pressure range of 3–4 MPa, and a Reynolds number range of 1 × 105–3.5 × 105. The Nusselt number and skin friction coefficient are calculated through control volume analysis proposed in this paper. Heat transfer enhancement and deterioration were observed in the experiments as well as the similar change of skin friction coefficient. The present results show that Chilton–Colburn analogy is also valid for turbulent flow and heat transfer of supercritical kerosene in horizontal straight circular tubes.

BEM Based Solution of Turbulent Flow Over Periodic Hills With Heat Transfer

2011 ◽  
Author(s):  
Leopold Sˇkerget ◽  
Jure Ravnik
Keyword(s):  
Heat Transfer ◽  
Turbulent Flow ◽  
Turbulent Flows ◽  
Stokes Equations ◽  
Navier Stokes ◽  
Small Scale ◽  
Test Case ◽  
Turbulent Structures ◽  
Eddy Simulation ◽  
Turbulent Models

Detached turbulent flows are difficult to predict numerically and often serve as benchmark cases for developing new numerical schemes and new turbulent models. Turbulent flow over periodic hills is one such examples, since the flow exhibits separation and reattachment on a smoothly and/or sharp curved geometry, strong pressure gradients and fluctuation of the separation point in time. These cases have been chosen by many authors for testing different turbulence simulation approaches. When the bottom wall is heated, the complexity of the problem increased, since convective heat transfer is defined by small scale turbulent structures close to the wall. We developed a Reynolds-Averaged Navier-Stokes and Large Eddy Simulation solver based on the velocity-vorticity formulation of Navier Stokes equations. RANS equations are coupled by a low-Reynolds number turbulent model, while Smagorinsky subgrid model is used for LES. The governing equations are solved with a numerical solution algorithm, which is based on the boundary element method. The pressure field is computed in a post processing step by solving a Poisson equation. The single domain as well as domain decomposition approaches are applied. The developed method was validated using flow over periodic hills test case.

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