Heated Parallel Channels Research Articles

Experimental investigation of the flow distribution of high-pressure CO2 flowing in heated parallel channels

This study experimentally investigated the flow distribution characteristics of high-pressure CO2 flowing in parallel heated pipes with an inner diameter of 2 mm. The test conditions were as follows: a system pressure of 7.5–9.5 MPa, an inlet mass flow of 200 g/min to 400 g/min, a heat flux of 55 kW/m2 to 400 kW/m2, and an inlet fluid temperature of 20–35 °C. The influences of working parameters on flow distribution were analyzed. The results showed that heat flux deviation between the parallel channels was the primary cause of the nonuniform flow distribution, and the flow deviation intensified with increasing heat flux deviation. Furthermore, a flow distribution model for high-pressure CO2 was established, which could effectively predict flow distribution in parallel channels, with an error of ± 1.5%. Thus, the results provide a theoretical foundation and technical support for the design and optimization of heat exchange systems in supercritical CO2 Rankine cycles.

CFD study of supercritical flow stability in two heated parallel channels with wall thermal mass effects

Three-dimensional (3D) numerical simulations were carried out of two vertical heated parallel channel experiments using ANSYS CFX to investigate the effects of wall energy storage on the system's stability boundary of a supercritical water up-flow system. Oscillatory instability boundaries of six experimental cases with two different wall thicknesses and uniform power distribution were produced, assessed and discussed. The 3D numerical solutions of the instability boundary with wall energy storage are in very good agreement with the experiments, which is believed to be a significant advance on previous work in this area. Results from a 1D linear code with wall thermal energy storage are also included for reference. The commonly used modelling assumption of circumferentially symmetric flow was found to be inadequate when modelling the wall heat storage. Temporal and spatial independence were ensured for the reported cases. The prediction of the oscillation periods obtained with CFX are also discussed.

Transient Flow Boiling and Maldistribution Characteristics in Heated Parallel Channels Induced by Flow Regime Oscillations

Flow boiling provides an effective means of heat removal but can suffer from thermal and hydrodynamic transients that compromise heat transfer performance and trigger device failure. In this study, the transient flow boiling characteristics in two thermally isolated, hydrodynamically coupled parallel microchannels are investigated experimentally. High-speed flow visualization is synchronized to high-frequency heat flux, wall temperature, pressure drop, and mass flux measurements to provide time-resolved characterization. Two constant and two transient heating conditions are presented. For a constant heat flux of 63 kW/m² into each channel, boiling occurs continuously in both channels and the parallel channel instability is observed to occur at 15 Hz. Time-periodic oscillations in the pressure drop and average mass flux are observed, but corresponding oscillations in the wall temperatures are virtually nonexistent at this condition. At a slightly lower constant heat flux of 60 kW/m², boiling remains continuous in one of the channels, but the other channel experiences time-periodic flow regime oscillations between single-phase and two-phase flow. At this condition, extreme time-periodic wall temperature oscillations are observed in both channels with a long period (~7 s) due to oscillations in the severity of the flow maldistribution. For the transient heating conditions, square-wave heating profiles oscillating between different heat flux levels are applied to the channels. Because of their relatively high frequency, the heating transients are attenuated by the microchannel walls, resulting in effectively constant heating conditions and flow boiling characteristics like that of the aforementioned constant heating conditions. This study illustrates the susceptibility of parallel two-phase heat sinks to flow maldistribution, particularly when undergoing transient flow regime oscillations.

Measurement of flow maldistribution induced by the Ledinegg instability during boiling in thermally isolated parallel microchannels

Flow boiling in a network of heated parallel channels is prone to instabilities that can cause uneven flow distribution, thereby degrading the heat transfer performance of the system and limiting predictability. This study experimentally investigates flow maldistribution between two parallel microchannels that arises due to the Ledinegg instability. The channels are heated uniformly and are thermally isolated from each other, such that both channels are subjected to the same input power regardless of the flow distribution. The channels are hydrodynamically connected in parallel and deionized water is delivered at a constant total flow rate shared by both channels. Direct measurements of the flow rate, wall temperature, and pressure drop in individual channels are performed simultaneously with flow visualization. At low power levels, when both channels remain in the single-phase liquid regime, the flow is evenly distributed between the channels and they attain the same wall temperature. As the power is increased, boiling incipience in one of the channels triggers the Ledinegg instability, which causes the flow to become maldistributed and induces a temperature difference between the channels. The severity of flow maldistribution, as well as the temperature difference between the channels, grows with increasing power. In the most extreme condition measured in this study, 96.5% of the total flow rate is directed to the channel operating in the single-phase liquid regime, while the boiling channel is starved and receives just 3.5% of the flow. The quantitative account of the flow maldistribution and temperature non-uniformity presented here provides a mechanistic understanding of the effects of Ledinegg instability-induced flow maldistribution on the heat transfer characteristics of thermally isolated parallel microchannels.

Experimental Investigation of Flow Instability Between Two Vertical Parallel Channels Using Supercritical CO2

Abstract Supercritical flow experiments were conducted at University of Manitoba using supercritical flow facility-vertical (SFF-V), which is a two vertical parallel channel assembly. The working fluid was CO2 at supercritical pressure and was driven by natural convective forces rather than a pump. Different system pressures (7.4 MPa–9.1 MPa), inlet temperatures (7 °C–30.1 °C) and various outlet-channel k-factors were used. A total of eleven parallel channel out-of-phase instability boundary points were obtained and the details of those cases are reported herein. These results can be used for code validation, to enrich the limited database of supercritical parallel-channel instability and to lend further insight into supercritical flow instability in heated parallel channels.

The effect of heat storage on supercritical flow stability in two heated parallel channels

One-dimensional (1-D) numerical simulations were carried out of two vertical heated parallel- channel experiments using a linear frequency domain approach to investigate the wall energy storage effect on the system's stability boundary for supercritical water up-flow. Oscillatory instability boundaries of the experimental cases with two different wall thicknesses, uniform and non-uniform power distributions are produced, assessed and discussed. In all, 31 cases were examined. These instability predictions with wall energy storage effects are compared with experimental data and other numerical results obtained without wall heat effects. There is improved agreement on the stability boundary predictions when the wall energy storage effect is included; however, all results fall within the experimental uncertainty. There is a cancellation effect of the wall energy storage in two parallel channels compared to a single channel. Consequently, the effect of wall energy storage is relatively small for the experiments modeled.

Ledinegg instability-induced temperature excursion between thermally isolated, heated parallel microchannels

Abstract Two-phase flow through heated parallel channels is commonly encountered in thermal systems used for power generation, air conditioning, and electronics cooling. Flow boiling is susceptible to instabilities that can lead to maldistribution between the channels and thereby heat transfer performance reductions. In this study, the Ledinegg instability that occurs during flow boiling in two thermally isolated parallel microchannels is studied experimentally. A dielectric liquid (HFE-7100) is delivered to the parallel channels using a constant pressure source. Both channels are uniformly subjected to the same power, which is in increased in steps. Flow visualization is conducted simultaneously with pressure drop, mass flux, and wall temperature measurements to characterize the thermal-fluidic effects of the Ledinegg instability. When the flow in both channels is in the single-phase regime, they have equal wall temperatures due to evenly distributed mass flux delivered to each channel. Boiling incipience in one of the channels triggers the Ledinegg instability which induces a temperature difference between the two channels due to flow maldistribution. The temperature difference between the two channels grows with increasing power until boiling incipience occurs in the second channel. The wall temperatures of both channels then reduce significantly as the flow becomes more evenly distributed. The experimentally observed temperature excursion between the channels is reported here for the first time and provides an improved understanding of the thermal performance implications of the Ledinegg instability in thermally isolated parallel channels.

Numerical study of oscillatory flow instability in upward flow of supercritical water in two heated parallel channels

A 3-D numerical study of turbulent flow of supercritical water flowing upward in two heated parallel channels with constant applied wall heat flux was developed using a RANS model in the Computational Fluid Dynamics (CFD) code ANSYS CFX. Oscillatory flow instabilities were investigated using the standard k-ε turbulence model with scalable wall functions. The effects of changes to the grid sizes and the time step size on flow instabilities were studied first. Then the instability thresholds of experimental cases were obtained with the CFX code. For comparison purposes, Chatoorgoon’s 1-D non-linear SPORTS code was also used to determine the instability boundary. These two new numerical results were compared with the experimental data and previous numerical results by other investigators. Additionally, the effects on the instability thresholds of changing the outlet plenum volume, the turbulent Prandtl number, the turbulence inlet conditions, the channel outlet K factor, the maximum iterations per time step in the transient analysis, and the order of the transient scheme were examined.

Non-linear stability analysis of uniformly heated parallel channels for different inclinations

The linear stability for two phase flow in parallel channels has been studied quite extensively. However, the analysis is valid only for infinitesimally small perturbations. Therefore, there is a need to carry out non-linear stability analysis for small finite sized perturbations. Moreover, earlier studies do not consider inclination of these channels, which exist for various applications. The focus of the present work is to carry out linear as well as non-linear stability analysis of parallel channels for various inclinations (including vertical and horizontal). The bifurcation analysis is carried out to capture the non-linear dynamics of the system and to identify regions in the parameter space for which subcritical and supercritical bifurcations exist. The study is carried out for different inclination angles in order to characterize the effect of inclination on the stability of the system. The analysis shows that, at all inclinations, a GH point and BT point exist. The subcritical and supercritical bifurcations are confirmed by numerical simulation of the time-dependent, nonlinear ODEs for the selected points in the operating parameter space using MATLAB ODE solver. The identification of these points is important because the stability characteristics of the system for finite (though small) perturbations are dependent on them.

Steady-State Performance of a Rectangular Natural Circulation Loop With Differentially Heated Parallel Channels

Natural circulation loop (NCL) transfers thermal energy without using any external power. As with phase change, one can expect a higher rate of heat transfer and a greater change in density, NCL with a phase change of the circulating fluid is a more effective energy transfer device. Though in many of the practical NCLs there are multiple heating risers, the characteristics of NCLs with parallel boiling risers have not been investigated in detail. In the present work, the steady-state behavior of a two-phase NCL with two parallel boiling risers for water as the working fluid has been investigated. Emphasis has been given to the performance of the loop when the risers are differentially heated. Effect of different parameters on the loop circulation rate and energy transport for both equally and differentially heated conditions has been thoroughly examined and compared to the performance of a single-riser loop under equivalent working condition.

An experimental investigation of flow instability between two heated parallel channels with supercritical water

Super critical water reactor (SCWR) is the generation IV nuclear reactor in the world. Under normal operation, water enters SCWR from cold leg with a temperature of 280°C and then leaves the core with a temperature of 500°C. Due to the sharp change of temperature, there is a huge density change in the core, which could result in potential flow instability and the safety of reactor would be threatened consequently. So it is necessary to carry out relevant investigation in this field.An experimental investigation which concerns with out of phase flow instability between two heated parallel channels with supercritical water has been carried out in this paper. Due to two INCONEL 625 pipes with a thickness of 6.5mm are adopted, more experimental results are attained. To find out the influence of axial power shape on the onset of flow instability, each heated channel is divided into two sections and the heating power of each section can be controlled separately. Finally the instability boundaries are obtained under different inlet temperatures, axial power shapes, total inlet mass flow rates and system pressures. The dynamics characteristics of out of phase oscillation are also analyzed.

Numerical simulation of the flow instability between two heated parallel channels with supercritical water

Supercritical water reactor (SCWR) is one of generation IV nuclear reactors recommended by Generation IV International Forum (GIF). Compared to traditional pressurized water reactor (PWR), SCWR has a lot of advantages and it is the most promising one among the six kinds of generation IV nuclear reactors. However there is a sharp change of water properties in the core of SCWR, especially the change of water density which could result in potential flow instability, consequently the safety operation of SCWR will be threatened. Therefore it is necessary to carry out relevant investigation in this field.In this paper, a three dimensional (3D) numerical simulation is carried out using the CFX code to investigate the out of phase oscillation between two heated parallel channels with supercritical water. The numerical procedure is discussed and proper numerical models such as mesh number, coupling method between velocity and pressure, time step, difference scheme and turbulence model are selected. Then, instability boundaries are obtained under different inlet mass flow rates, system pressures, with and without gravity conditions. Because different physical models are adopted in the 3D simulation, the instability boundaries differ from those obtained with a 1D code. Finally, the numerical result is compared with experiment data, which shows that the 3D code could predict the onset of flow instability better than the 1D code. However there should be a further improvement of the physical models in the 3D code due to its inadequacy to predict the period of oscillation.

Laminar flow instability in multiple channels

Temperature-viscosity-induced laminar flow instability (LFI) in two gaseous heated parallel channels with interchannel heat exchange is purely excursive, rather than oscillatory. Constant total flow always leads to a stable system, while constant pressure drop could have an instability, depending on the sign of ( ∂ΔP/ ∂W) Q . The system was studied numerically with negative heat perturbations yielding bounded excursions, and positive heat perturbations giving unbounded excursions asymptotically approaching zero. A nine-channel system was probed, giving excursive behavior with the ultimate growth rate the same for single and multichannel systems.