Horizontal Cooler Research Articles

Computational and analytical study on the stability and utility aspects of single-phase natural circulation loop in parabolic trough collector

The use of passive heat transport devices in solar thermal energy devices is critical to make the systems more reliable and sustainable. Hence, linking single phase natural circulation loop to parabolic trough collector is the apt solution. In this regard, a 3-D computational model is built and validated for steady state and transient results. To acquire realistic results, solar irradiance pattern over the receiver is explored from SolTrace® and applied to the loop. The upper threshold of the horizontal heater horizontal cooler loop is found and validated using linear stability analysis. Since the loop is unstable at lower loads, instability restraining techniques viz. loop inclination (θ = 4°) and orifice insertion (β = 0.7) are employed. Although loop inclination gave desired results, the orifice couldn't improve the behavior due to loop geometry. The type of heat flux pattern does affect the initial transients of the loop, but it does not affect the steady state results in any manner. The loop was also tested with Therminol VP 1 and compared the stability aspects with water. In the end, intending NCL application in PTC, transient analysis was undertaken considering whole day solar flux distribution, which gave convincing results.

ДОСЛІДЖЕННЯ ЕФЕКТИВНОСТІ ТЕПЛООБМІННОГО ПРОЦЕСУ В ОХОЛОДЖУВАЧІ ТУРБІННОГО МАСТИЛА МО-2 В ЗАЛЕЖНОСТІ ВІД СХЕМИ РОЗТАШУВАННЯ ТРУБОК У ТРУБНИХ ҐРАТАХ

Effective cooling of the turbine oil is important for the reliability and optimal operation of the turbogenerator. The oil is used for lubrication and cooling of the support and radial bearings of the turbine, bearings of the generator and reducer, gears and splined rollers. It is also used as a hydraulic fluid in the hydraulic turbine start-up system. For reliable and safe operation of the system, it is necessary to maintain the correct temperature of the lubricant. Research is aimed at studying the influence of the layout of the heat exchange tubes in the tube grids on the cooling efficiency of the TP-22 turbine oil. With the help of the SolidWorks software complex, a geometric model of the MO-2 turbine oil cooler was created with three schemes for the placement of heat exchange tubes in tube grids: corridor, radial and checkerboard. Adhering to the parameters of the technical characteristics of this cooler, the nominal parameters of temperatures and consumption of coolants (cooling water and turbine lubricant TP-22) were chosen. The result of the calculations were the temperature fields of the coolants and the trajectories of the movement of lubricant particles in the intertube space of the cooler. When creating a geometric model and further research, the main technical characteristics of the shell-and-tube horizontal cooler MO2 were adopted, namely: the temperature of the turbine oil at the inlet is 45°С, the nominal flow rate of the oil is 3.1 m3/h, the temperature of the water at the inlet is 20°С, the nominal flow rate water - 15 m3/h. The conducted computer modeling made it possible to investigate the influence of the layout of the heat exchange tubes in the tube grids on the cooling efficiency of the turbine lubricant in the MO-2 shell-and-tube cooler. It was shown that the most effective, among the studied schemes, is the checkerboard arrangement of heat exchange tubes in tube grids. This scheme makes it possible to reduce the consumption of cooling water, relative to the nominal one, which will allow the use of water pumps of lower power.

Experimental and analytical study on the stability of a low aspect ratio single-phase natural circulation loop coupled to a parabolic trough collector

Use of passive heat transport systems in renewable energy eliminates operator and external power requirements, thereby reducing the cost. Hence, a horizontal heater and horizontal cooler rectangular natural circulation loop (NCL) of low L/D (110) and aspect ratio (0.54) is coupled to a parabolic trough in the present work. Experimental investigations of the loop's steady state and stability characteristics were carried out at various powers corresponding to the solar irradiances using water as the heat transfer fluid. SolTrace was used to determine the heat loads at different direct normal irradiance (DNI) for the selected loop and trough dimensions. The upper threshold for stability was experimentally observed for the first time and reported here. Stability analysis carried out for different loop diameters revealed that the power at which the upper threshold of instability occurs reduces significantly with increase in loop diameter, enabling its observation in the low-pressure low temperature loop employed in the present study. Stabilizing techniques like orifice insertion and loop inclinations were also studied in detail. The results showed stability at higher heat loads (> 350 DNI), while the instability at lower heat loads can be curbed using an orifice (β = 0.5) or a loop inclination (> 4°). The small inclination can eliminate multiple steady state solutions and hence stabilizes the loop. Besides, the reduction in natural circulation flow rate with inclination is much smaller compared to the orifice making its heat transport capability higher. Additionally, the steady state results were validated with 1-D analytical model giving a good match.

Steady state performance of molten salt natural circulation loop with different orientations of heater and cooler

Molten salts are proposed as a coolant and sensible heat storage medium for Solar Thermal Power Plant (STPP) as well as fuel and coolant for Molten Salt Reactor (MSR). The objective of the present study is to investigate the steady state behavior of molten salt in natural circulation flow condition along with effect of different orientations of heater and cooler on the steady state mass flow rate. For which a Molten Salt Natural Circulation Loop (MSNCL) has been setup with molten nitrate salt (mixture of Sodium Nitrate and Potassium Nitrate in 60:40 ratio by wt.) as working fluid. Steady state experiments in four different orientations of heater and cooler [Horizontal Heater Horizontal Cooler (HHHC), Horizontal Heater Vertical Cooler (HHVC), Vertical Heater Horizontal Cooler (VHHC) and Vertical Heater Vertical Cooler (VHVC)] have been performed and data generated are compared with the steady state correlations available in open literature which are derived considering either standard friction factor correlations or friction factor correlations deduced from their experiments and theoretical studies. Based on the deviations observed in the experimental values and correlation predictions, an improved formulation has been derived. The steady state formulation has been improved by incorporating Wall Thermal Inertia (WTI) effect and Heat Loss (HL) effect considering the importance of WTI & HL in dealing with high temperature molten salt loop. The improved formulation has been compared with the experimental results for different orientations of heater and cooler. Further, a new friction factor correlation has also been deduced from the experiments for vertical heater orientation. Description of MSNCL along with validation studies performed for different steady state correlation against experimental data generated in MSNCL, are reported in this paper.

Experimental Investigations on Heat Transfer Deterioration for Vertical Flow of Supercritical Carbon-Dioxide Under Natural Circulation

Abstract Many upcoming designs of nuclear reactors conceptualize use of supercritical fluids (SCFs) due to certain associated advantages. Supercritical helium (SC-He), supercritical carbon-dioxide (SC-CO2), and supercritical water (SCW) find proposed applications in primary/secondary heat removal cycles in nuclear reactors. The knowledge of heat transfer characteristics of the working fluid is essential prerequisite for its actual application. Similarly, for SCF, heat transfer deterioration (HTD) is an area having much ambiguity and thus has attracted many researchers in the recent past. Still, conclusive guidelines for design applications of SCF are lacking due to dramatic variation of thermos-physical properties of SCF in the pseudo-critical region. In this paper, extensive experimental investigations are undertaken to study HTD of SC-CO2 under natural circulation (NC) system. Experiments conducted at pressures between 7.9 and 12.31 MPa for vertical heater horizontal cooler (VHHC) and vertical heater vertical cooler (VHVC) are reported. Effect of heat flux, operating pressure, and cooler orientation are discussed based on the experimental data generated. Methodologies available in literature for identification of HTD are also examined with the present experimental data. The experimental data are compared with the HTD criteria available in literature and a new criterion for onset of HTD is proposed for vertical flows under natural circulation. Role of buoyancy (Bu) and acceleration (Ac) parameters are also investigated. Noticeable HTD is observed for Bu > 3 × 10−6 whereas Ac is not found to affect the HTD in this study.

Computational fluid dynamics analysis of the natural circulation system in vertical heater horizontal cooler (VHHC) molten salt loop

Molten salt has attracted the attention of researchers, especially as a coolant because of its characteristics. This study aimed to analyse the transient and steady-state of the natural circulation system in the molten salt loop with vertical heater and horizontal cooler using computational fluid dynamics. The computational fluid dynamics module in the COMSOL Multiphysics software was used to solve governing equations. Experimental data from several references were used as input data in modelling. The loop was modelled as a rectangular loop with Inconel 625 as the pipe material and molten salt (NaNO3+KNO3) as the working fluid. The variation of heater power was also carried out in modelling. The modelling results showed a good agreement with the experimental results. The results provided that the molten salt natural circulation loop with vertical heater and horizontal cooler showed good flow stability without reversal of flow direction.

Stability performance of series coupled natural circulation system with different operating procedures

Series coupled natural circulation system with the last loop being an open-air loop finds application as an infinite mission time passive decay heat removal system in nuclear power plants and solar thermal systems. Hence the stability behavior of series coupled rectangular loops with horizontal heater and horizontal cooler orientation was experimentally studied. The loop was subjected to various operating procedures viz., start up from rest, power raising from a stable steady state and power step down from an unstable state to witness that the stability threshold was not a unique value, thus confirming the existence of a hysteresis phenomenon. The stability threshold enhancement of the system with orifice plates was maximum for the lowest orifice diameter. The coupled system was chosen with an unstable first loop and the instability was found to transmit from loop#1 to the subsequent loops. However, the oscillation amplitude was highly attenuated and seemed to be nearly stable in the 2nd and 3rd loop without any flow reversals even when the first loop was unstable with chaotic bidirectional pulsing. In case of bi-directional pulsing in loop#1, the heat exchanger between the first two loops operated alternatively in co-current and counter-current mode with no change in the flow direction of loop#2. A delay in the flow initiation was found for start-up from rest, which increased from the first loop to the last loop and decreased with the increase in power input. Further, the numerical simulation with the one-dimensional model reproduced the oscillatory behavior in loop#1, with the predicted stability threshold being considerably lower compared to the experiments. The attenuation of the oscillations in the loop#2 as observed in the experiments could be reproduced. Predicted transients showed practically no oscillations in loop#3 corroborating the experimental observations. The attenuation of the oscillations in loop#2 and the absence of oscillations in loop#3 were attributed to the damping caused by the wall and also due to the higher Lt/D ratio in loop#2 and a vertical air flow channel in loop#3 respectively. The phase plots indicated a near periodic oscillation compared to the highly chaotic pattern in the experiments, presumably due to the higher power at which instability was observed in the experiments. The deviations were also due to the secondary flows being generated in the large diameter loop as the simulation was based on 1D along with the heat loss being neglected.

Stability and thermal analysis of a single-phase natural circulation looped parabolic trough receiver

The sustainability and reliability of solar parabolic trough technology can be enhanced by introducing a natural circulation loop. The configuration that suits better is horizontal heater horizontal cooler, which is the most efficient but least stable loop. Hence, as an effort towards the evolution of natural circulation looped collector, 3D computational analysis of a 70 mm uniform diameter square loop (Dowtherm A as the heat transfer fluid) is done with uniform and non-uniform heat flux (SolTrace based) as well as different instability restraining techniques. Prior to the analyses, the stability maps were generated and validated, which predicts high level of instability. Later, the validated computational model is used for parametric analyses, which includes heat flux pattern, loop tilt, use of orifice, and combination of tilt and orifice. Since the loop geometry is highly unstable (ratio of loop length to diameter is 57), the stable operation is observed only with high tilt (θ = 45°), low beta ratio (β = 0.25) and combinations of tilt and orifice (β (0.50) + θ (15°) and β (0.50) + θ (30°)). However, the latter method is suggested for trough application as the former two versions are impractical. Further, the heat load analysis with uniform and non-uniform flux designated insignificant variation in system performance. Since the loop is effective with restraining techniques and its performance is on par with the conventional trough system (predicted by the analytical model), an effort towards implementation of natural circulation loop in parabolic trough technology is prolific.

Stability enhancement in large diameter rectangular natural circulation loops using flow restrictors

Single-phase natural circulation loops (SPNCL) finds application in district heating reactors and low-power pressurized water reactors. It is not only adopted as a backup operation mode during accidental scenarios but also as the normal operating mode in several small and medium-sized advanced nuclear power plants. Stable operation is desired in such plants for smooth power generation and hence, the stability analysis of single-phase natural circulation systems assumes significance. The horizontal heater and horizontal cooler combination produce a maximum flow rate for a specific heater power but also is the most unstable compared to other configurations. Out of many instability restraining techniques, the orifice plate has been targeted by very few. The present experimental study focuses on the effect of using orifice plates (β = 0.90, 0.77, 0.64, and 0.51) on the stability of the 38.89 mm diameter rectangular loop with a horizontal heater and cooler configuration. The various operating procedures (start-up from rest, power raise from a stable steady-state, and power step down from an unstable state) have shown different stability/instability thresholds. Further, a hysteresis phenomenon was observed between the operating procedure of power raise from a stable state and the power step down from an unstable state, both exhibiting different stability thresholds. The increase in the stability threshold for β = 0.9, 0.77, and 0.64 for the operating procedure of start-up from rest was 75%, 110%, and 225% respectively compared to without orifice. Since the heater power was limited to 1100 W, the orifice of β = 0.51 was seen to be completely stable for the entire range of all three operating procedures for the same cooler conditions. From the loss coefficient (K), the effective length of the loops for β = 0.9, 0.77, 0.64, and 0.51 increased by 0.36, 1.32, 4.72, and 21.8 m respectively. Accordingly, the steady-state analytical solution with and without orifice found a good agreement with the experimental results. Thus, the present experimental investigations show that with the judicious choice of the flow restrictor, an unstable natural circulation system can be stabilized although the steady-state flow decreases.

Mathematical modeling and numerical investigation of density wave instability of supercritical natural circulation loop

In present paper, a mathematical model based on the one dimensional nonlinear mass, momentum and energy conservation equations has been developed to study the density wave instability (DWI) in horizontal heater and horizontal cooler supercritical water natural circulation loop (HHHC-SCWNCL). The one dimensional nonlinear mass, momentum and energy conservation equations are discretized by using finite difference method (FDM). The numerical model is validated with the benchmark results (NOLSTA model). Numerical simulations are performed to find the threshold stability zone (TSZ) and draw the stability map for natural circulation loop. Further, effect of change in diameter and riser height on the density wave instability of SCWNCL has been investigated.

Investigations on the dependence of the stability threshold on different operating procedures in a single-phase rectangular natural circulation loop

During the past years, several studies were focused on the instability of single-phase rectangular natural circulation systems. However, experimental identification of instability threshold and categorization of instability regimes is of prime interest in many industrial applications which have been addressed by very few researchers that too limited to lower diameter loops only. Hence, in the present study, a larger diameter rectangular loop with horizontal heater horizontal cooler (HHHC) configuration has been designed to generate single phase natural circulation data on the steady state and stability behavior. Effect of different operating procedures (i.e., start-up from rest, power raising from stable steady state, and power reduction from an unstable state) on stability behavior are studied. Most of the oscillatory regimes are identified and noticed that the operating procedure can influence the instability threshold. The instability threshold was found to increase with the lowering of the coolant temperature and increasing its flow rate. The operating procedures i.e., power raising from stable steady state and start-up from rest gave the maximum and minimum values of the stability threshold respectively. As opposed to this, the minimum instability threshold in smaller diameter loops was reported for the operating procedure with power reduction from an unstable state. The experimental results confirm the occurrence of the hysteresis phenomenon. Due to the larger diameter and lower Lt/D ratio of the loop, all the oscillatory behaviors were aperiodic in nature in contrast to the observations in smaller diameter loops.

Modification of SIMPLE algorithm to handle supercritical natural circulation in a loop

The conventional SIMPLE algorithm for the pressure–velocity coupling has been adopted by many commercial CFD codes. Since it encounters convergence problem when it is used for numerical analysis of a two-dimensional unsteady natural convection flow in a rectangular cavity with zero-isothermal compressibility, the modification of SIMPLE algorithm has been proposed for such a fluid. In this paper, differences between solutions for a one dimensional natural circulation of a super-critical carbon dioxide in a loop with a horizontal heater and a horizontal cooler configuration obtained by using the conventional SIMPLE algorithm and the modified version of the algorithm are investigated. The modification of the algorithm includes updating the density at each time step based on its value at the previous time step to satisfy the mass conservation. The differences of the velocity and temperature are relatively small comparing with the two-dimensional natural convection case. As an example of utilizing the modified SIMPLE algorithm, characteristics of the unsteady natural circulation of super-critical carbon dioxide in a rectangular loop are revealed.

The influence of the wall thermal inertia over a single-phase natural convection loop with internally heated fluids

This paper deals with the influence of the piping material thermal and geometrical properties on the dynamic stability of single-phase natural circulation loops. To this purpose, a semi-analytical approach is developed by adopting the tools provided by the linear analysis. By considering a generic natural circulation loop configuration with a localized heat flux and a homogenously distributed Internal Heat Generation (IHG), the governing equations (mass, momentum and energy balance) are linearized around a steady-state solution of the system and treated by means of the Fourier transform to obtain dimensionless stability maps. Moreover, in order to verify the linear analysis methodology, a numerical strategy is adopted to solve the nonlinear governing equations and to investigate the natural circulation dynamics in the time domain. In principle, both the developed approaches can be applied to any natural circulation loop configuration. In the present work, the linear and the nonlinear analyses are applied to a specific natural circulation loop geometry, namely the Horizontal Heater Horizontal Cooler (HHHC) one. In this regard, an Object-Oriented (O-O) one-dimensional model of the HHHC loop is developed. For the assessment of the O-O model, the obtained results are compared with RELAP5 and Computational-Fluid-Dynamics (CFD) time-dependent simulations.

Analytical and numerical investigation of the heat exchange effect on the dynamic behaviour of natural circulation with internally heated fluids

In this paper, the study of the heat exchange effect on the dynamic behaviour of single-phase natural circulation with internal heat generation is presented. In order to predict natural circulation instabilities, two different methods of analysis are developed and compared. The first approach is a linear analysis in which the governing equations are firstly linearized around a steady-state solution of the system and then treated by means of the Fourier transform. This strategy is adopted to compute, in a semi-analytical way, dimensionless stability maps for different system configurations, highlighting the heat exchange effect on the system dynamics. The second approach consists in numerically solving the nonlinear governing equations and allows investigating some transients of interest. For this purpose, an object-oriented one-dimensional model of natural circulation loops has been developed, and the corresponding results have been compared with RELAP5 and Computational Fluid-Dynamics (CFD) time-dependent simulations. The developed models have been applied to investigate the dynamic behaviour of two loop configurations characterized by large instability regions, namely the Horizontal Heater–Horizontal Cooler (HHHC) and the Vertical Heater–Horizontal Cooler (VHHC).

Investigation on Heat Transfer Behaviour of Molten Salt Natural Circulation Loop using Numerical Simulations

Molten salts are used as a coolant/heat transfer fluid in various high temperature engineering systems owing to their high boiling point at low pressure. Natural circulation of molten salt is being preferred in some systems like solar thermal power plant or in some nuclear reactors. Such systems can be studied with the help of a natural circulation loop. In this work, heat transfer characteristics of Molten Salt Natural Circulation Loop (MSNCL) are studied using 3D CFD simulations. Molten Nitrate salt, NaNO3+KNO3 (60:40 ratio by weight), is used as a fluid in MSNCL. In the MSNCL, in heater section, flow is developing and also mixed convection flow regime exists. The local Nusselt number variation in heater is calculated from computed data and is compared with that from Boelter correlation. Steady state heat transfer characteristics are obtained over a wide range of Reynolds number using CFD simulations. Unsteady heat transfer characteristics in the oscillatory flow formed in MSNCL with horizontal heater configuration are also studied and are found to be different as compared to vertical heater placed in vertical heater horizontal cooler configuration.

Computational study of instabilities in a rectangular natural circulation loop using 3D CFD simulation

Steady state and transient characteristics of a natural circulation loop working with water are obtained. For this purpose, 3D steady state and transient CFD simulations are performed. The CFD model includes pipe thickness as well as secondary side coolant passage apart from primary side. Steady state and transient characteristics are computed for various configurations i.e. Vertical Heater Vertical Cooler (VHVC), Horizontal Heater Horizontal Cooler (HHHC), etc. Steady state data was compared with available correlations. Flow initiation transients were compared with experimental data. Both the steady state and transient results are found to be in good agreement with previously published data. The reason for formation of unidirectional and bi-directional pulsing in HHHC configuration at different powers is explained with the help of temperature fields at different instants of time. Effect of sudden power rise/power step back on instability in HHHC configuration is estimated using CFD simulations.

MANUFACTURE AND PERFORMANCE EVALUATION OF A HORIZONTAL SINGLE-PASS ANIMAL FEED PELLETS COOLER

The present research was carried out to manufacture and evaluate the performance of a horizontal single-pass animal feed pellets cooler to improve product quality. The cooler performance was studied as a function of change in pellets feed rate, conveyor chain speed and airflow rate. Performance evaluation of the horizontal cooler was carried out in terms of final pellets moisture content, cooling efficiency, pellets durability, energy requirements and cooling cost. The experimental results revealed that final pellets moisture content, cooling efficiency, pellets durability, energy requirements and cooling cost are in the optimum region under the following conditions: - Pellets feed rate of 2.2 Mg/h, - Conveyor chain speed of 2 cm/s, - Airflow rate of 12 m3/s.

Investigations on single-phase natural circulation loop dynamics part 1: Model for simulating start-up from rest

Single-phase natural circulation loops are used in many industrial systems like nuclear reactors, geothermal systems, solar water heating systems and process industry. The present paper presents a 1-D model for simulating the startup from rest of water cooled single-phase natural circulation loops. A pseudo-conductivity model was developed previously by the authors to account for heat diffusion caused by local convection currents and the results of numerical simulation for loops having horizontal heater and horizontal cooler were presented. In the present study, the model has been used to simulate the start-up of loops having different heater and cooler orientations and the applicability of the model as a general tool for simulating start-up of rectangular natural circulation loops is assessed. The paper presents more insight into the behavior of pseudo-conductivity models developed to simulate start-up of these loops from state of zero flow.

Natural convective flow and heat transfer studies for supercritical water in a rectangular circulation loop

A closed Supercritical Pressure Natural Circulation Loop (SPNCL) has been set up to generate data on flow and heat transfer behavior for supercritical fluids (i.e. water and carbon dioxide) under natural circulation conditions. Experiments were conducted in SPNCL with water as working fluid. Instability was observed over a narrow window of power in the pseudo-critical temperature region for horizontal heater horizontal cooler orientation. Instability was observed at low supercritical pressure range of 22.1–22.9MPa, however it got suppressed at higher pressures. An in-house developed computer code NOLSTA has been used to perform steady state and stability analysis of SPNCL and predictions are compared with experimental data. The heat transfer data generated in the test facility has been compared with various heat transfer correlations for supercritical fluids available in literature. The present paper describes the experimental results and comparisons in detail.

Steady state and stability characteristics of natural circulation loops operating with carbon dioxide at supercritical pressures for open and closed loop boundary conditions

Experiments were conducted in a closed supercritical pressure natural circulation loop (SPNCL) with carbon dioxide as working fluid. Instability was observed in a narrow window of power with the loop operating in the pseudo-critical temperature range (heater inlet temperature in the range of 27–31°C and heater outlet temperature oscillating from 29 to 45°C). Only horizontal heater horizontal cooler orientation exhibited instability and that too at low secondary side chilled water mass flow rate, i.e. 10–15lpm. A computer code NOLSTA has been developed to carry out steady state and stability analysis of open and closed loop natural circulation at supercritical conditions. The code adequately predicts steady state natural circulation performance for both open and closed loop boundary conditions. However, code predicts very large unstable zone for SPNCL, hence pipe wall thermal capacitance models were incorporated in NOLSTA code to simulate the experimental results. The stability predictions by NOLSTA code (with pipe wall thermal capacitance model) have been compared with experimental data available in open literature and experiments conducted in SPNCL.