Performance Of Natural Circulation Loop Research Articles

Analysis of the heat-carrying performance of natural circulation loop using modified RELAP5/MOD3.4

Since the successful manufacture of the separate heat pipe (also known as Two-phase Loop Thermosyphon or Natural Circulation Loop), it has been used in many applications, especially for waste heat recovery/remove and cooling systems. We use the modified system analysis program RELAP5 to study the medium-sized separated heat pipe setup MSL2 built by Seok-Ho Rhi. To better calculate heat pipe performance under low pressure, we have improved a series of models, including the bubble nucleation model, the heat flux partition model, the interfacial heat transfer, the drift flux model, the wall heat transfer model, etc. Then we use the low-pressure subcooled boiling experiment to verify the modified code. The validation results show that the modified code accurately captures the subcooling boiling phenomenon under low pressure and low flow conditions. We use the modified code to investigate the factors influencing heat-carrying performance and analyze their action mechanisms. The results show that heating power, volumetric filling rate, and cooling water temperature influence heat-carrying performance. Additionally, we analyze the heat pipe operating parameters over time to obtain the characteristics of the heat pipe at various stages.

CFD study on the effect of nanofluids in natural circulation loop

A vertically placed natural circulation loop consists of a heater in the riser pipe and a heat exchanger at the top end for heating and cooling the working fluid. In this investigation, the heat source is placed in the side pipe of the loop called riser. The performance of natural circulation loop is investigated with water, Alumina nanofluid and Copper Oxide (CuO) nanofluid of 0.5%, 1% and 1.5% (by wt) concentration. The behaviour of the loop for various heat inputs at the heat source is investigated with water and then with Alumina and CuO nanofluids.It is found that the performance of the loop is high when the working fluid used is copper oxide nanofluid compared to alumina nanofluid and water. The mass flow rate is highest for 1.5% CuO nanofluid which is 4.58 g/s. Heat transfer coefficient of the loop fluid at the heat source increases with an increase in the heat input at the heat source. There is also a significant difference for heat transfer coefficients of water and the nanofluids of CuO and alumina in the natural circulation loop for various amounts of heat supplied in the heat source.

Thermal performance of natural circulation loop filled with Al2O3/Water nanofluid

Natural Circulation Loop (NCL) which is also called as a thermosyphon system is the heat transfer loop which uses no pump or external device to drive the loop fluid. In the present paper, a comparative study on thermal characteristics of two loop fluids viz. water and Al2O3/water nanofluid is made. Experiments are conducted on in-house designed test rig. Thermo-hydraulic behaviour of loop fluid is presented. Two parameters such as heat input, nanofluid concentration are varied in order to study their individual and combined effects. It is concluded that Al2O3/water nanofluid as loop fluid results in higher mass flow rates as compared to the water. Different derived quantities such as Nusselt number and Grashof number are calculated. Quantitative comparison is made between water and Al2O3/water nanofluid. Time to reach steady state is reduced by 22 % using Al2O3/water nanofluid as loop fluid when compared with water. Mass flow rate and Grashof number of the Al2O3/water nanofluid based NCL are enhanced by 6.75% and 26% respectively, when compared with water-based NCL at 1000W heat input. At the heater, the temperature gradient is reduced by 30.2% due to the improved thermal and transport properties of Al2O3/water nanofluid when compared with water at 1000 W heat input. As particle concentration increases from 1% to 5%, Nusselt number increases from 10.1 to 20.1, for the heat input of 1000W.

Plenum-to-plenum natural convection heat transfer within a scaled-down prismatic modular reactor facility

Multiphase Reactors Engineering and Applications Laboratory (mReal) at Missouri University of Science and Technology (S&T) has developed a natural convection heat transfer test facility with one riser and one downcomer between two plena to investigate loss of flow accident scenario (LOFA) for a prismatic very high temperature reactor (VHTR). Using advanced heat transfer coefficient probe and T-thermocouples (1.6 mm), this paper reports the effect of the outer surface temperatures of the upper plenum and downcomer channel (278.15, 288.15, 298.15, and 308.15 K) on the intensity of natural circulation during LOFA with helium at 413.685 kPa. The results showed that there is a reduction in the centerline temperature of ∼10.14% and inner wall surface temperatures of ∼7.4% along the riser channel with decreasing upper plenum and downcomer temperatures from 308.15 K to 278.15 K. A reversal in the direction of heat transfer is observed close to the exit of the riser channel (Z/L = 0.773) for outer surface temperature 288.15, 298.15, and 308.15 K due to the end effect. It is worth mention that, the negative signals of heat fluxes are observed along the downcomer channel for all operating conditions, which confirms heat removal from helium and a downward flow along the downcomer channel, hence establishment of natural circulation. The results also showed a gain in the values of heat transfer coefficients along the riser channel with decreasing the outer surface temperature which is consistent with the literature. In comparison to the literature with air as coolant, the current results showed the role of helium on the thermal performance of natural circulation loop in terms of co-circulation of plumes and end effects.

Numerical study on heat transfer characteristics of nanofluid based natural circulation loop

In this paper the steady-state analysis has been carried out on single phase natural circulation loop with water and water based Al2O3 (Al2O3-water) nanofluid at 1%, 3%, 5%, and 6% particle volume concentrations. For this study, a 3-D geometry of natural circulation loop is developed and simulated by using the software, ANSYS (FLUENT) 14.5. Based on the Stokes number, mixture model is adopted to simulate the nanofluid based natural circulation loop. For the simulations, the imposed thermal boundary conditions are: constant heat input over the range of 200-1000 W with step size of 200 W at the heat source and isothermal wall temperature of 293 K at the heat sink. Adiabatic boundary condition is imposed to the riser and down-comer. The heat transfer characteristics and fluid-flow behavior of the loop fluid in natural circulation loop for different heat inputs and particle concentrations are presented. The result shows that the mass-flow rate of loop fluid in natural circulation loop is enhanced by 26% and effectiveness of the natural circulation loop is improved by 15% with Al2O3-water nanofluid when compared with water. All the simulation results are validated with the open literature in terms of Reynolds number and modified Grashof number. These comparisons confidently say that the present 3-D numerical model could be useful to estimate the performance of natural circulation loop.

Assessing the effect of flashing on steady state behavior and Ledinegg instability of a two phase rectangular natural circulation loop

In the present work, we propose to study the effect of ignoring flashing on the performance of a natural circulation loop operating under steady state. To this end, the performance of a rectangular natural circulation loop with water as the working fluid has been simulated considering both homogeneous equilibrium model (HEM) and drift flux model (DFM) for the two-phase zone. It has been observed that flashing has a substantial effect on the performance of the loop in certain operating zones and one may even experience an anomaly in the predicted value of local temperature if flashing is neglected. Flashing, in general, reduces the circulation rate under steady operating condition. But at the same time it makes the operation of the loop more stable against Ledinegg instability. In general, the effect of flashing is predominant at low pressure and low heat flux.