Secondary Fluid Flow Research Articles

Fracture attribute and topology characteristics of a geothermal reservoir: Southern Negros, Philippines

The characterization of fracture networks using attribute and topological analyses has not been widely applied to the understanding and prediction of the secondary porosity, permeability and fluid flow characteristics of geothermal resources. We acquired fracture length, aperture, intensity and topological data from remotely sensed images and surface exposures of the Cuernos de Negros region and compared these data with well cores and thin sections from the underlying active geothermal reservoir: the Southern Negros Geothermal Field, west central Philippines. We show that the fracture attributes of the analogue and reservoir are best described by a power law distribution of fracture length and aperture intensity across six to eight orders of magnitude. This characterization of outcrop and borehole fractures validates the use of surface exposures as analogues for the Southern Negros Geothermal Field reservoir rocks at depth. An observed change in the scaling exponent in the 100–500 m length scale suggests that regional to sub-regional fracture systems scale differently from those at the meso- and macroscale, which may be a strata-bound effect or a sampling issue. Topological analyses show a dominance of Y-nodes and doubly connected branches, that indicates a high degree of fracture connectivity, which is important for effective fluid flow. Supplementary Material: Slopes, coefficient of determination and Aikake information criterion values of the cumulative frequency v. length and aperture plots of all fracture transects are available at https://doi.org/10.6084/m9.figshare.c.4960559 Thematic collection: This article is part of the The Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs

New Approach of Triumphing Temperature Nonuniformity and Heat Transfer Performance Augmentation in Micro Pin Fin Heat Sinks

Abstract This study is the first part of the development of improved micro pin fin heat sink (MPFHS) for the thermal management of modern microprocessor chip cooling. In the current numerical study, a new fluid flow distribution scheme for MPFHS has been proposed for triumphing over surface temperature nonuniformity problem—one of the most critical issues interfering with the thermal management of modern microprocessors chip cooling. It is established that fluid, if supplied from the confronting sides (front/side directions) of the MPFHS, helps in mitigating temperature nonuniformity and intensifies heat transfer rate. Fluid starts enjoying following paybacks on account of proposed change: the benefits of the developing flow even in adverse temperature zones of the conventional design, enriched secondary channels fluid flow, and rigorous mixing of the cooling fluid between the primary and the secondary channels. Two front facing multi-inlet designs (MPFHSMI,F and MPFHSMI,FH) and one side facing multi-inlet design (MPFHSMI,SH) are conceptualized and compared with the conventional design MPFHSCD. Base surface temperature nonuniformity reduces 7.6 °C, 24 °C, and 7.4 °C by the MPFHSMI,F, MPFHSMI,FH, and MPFHSMI,SH designs, respectively. Average Nusselt number for the cases MPFHSMI,F, MPFHSMI,FH, and MPFHSMI,SH is found 26.7%, 52.3%, and 70.9% higher than the conventional design of MPFHS. Overall thermal performance factor of one design MPFHSMI,FH is found 1.66 at the applied heat flux of 125 W/cm2.

Performance investigation of a novel calorimeter for a heat pump system according to flow loops

Heat pumps cover a wide range of heating and cooling applications owing to their means of drawing out heat from ground, air and water at low temperature. In order to acquire performance data for heat pump certification and rating, the calorimeter is used in measuring the heat pump. In conventional approach of measuring the heat pump with standard test conditions, a great amount of energy is used in the calorimeter for cooling and heating. Using innovative ways to enhance the calorimeter performance is significant for energy and cost savings. Therefore, a novel calorimeter was developed for heat pump measurement. Two different flow loop configurations, specified as cases (1) and (2) were adopted. In the case (1), secondary fluid flow through the heat recovery unit was directly from heat pump test unit heat exchanger while in case (2), secondary fluid flow through heat recovery unit was from constant temperature water bath directly. The energy usage of the novel calorimeter based on the impact of each configuration was analyzed. Experiments were conducted using a water-to-water heat pump unit in heating and cooling approach with variation in operating conditions. Energy analyses based on test data indicated that energy reduction of the novel calorimeter was at least 72 % for heating and 69 % for cooling in relation to energy used in conventional calorimeter. Moreover, for novel calorimeter, case (1) resulted in energy savings of at least 9.1 % for cooling and 13 % for heating in relation to case (2). However, for heat pump units with very low capacities, tests measurements were executed using case (2) configuration because the tests in case (1) resulted in large thermal fluctuations such that the steady operating conditions of the water setpoint temperatures at inlet to the indoor and outdoor heat exchangers of the test unit were unattainable.

Numerical Investigation of the Flow Dynamics Inside Supersonic Fluid Ejector

This paper describes the flow dynamics inside a supersonic ejector using CFD modelling. Suitable ejector geometry is proposed for the high compression ratios encountered in real-world applications. Post-processing and physical analysis of the CFD results are presented to better understand the entrainment mechanism and the mixing between the primary and the secondary fluids. The effect of the primary pressure on the flow dynamics and the entrainment ratio is discussed for the two compression ratios of 1.67 and 3.4. It is found that the proposed ejector can operate at compression ratio as high as 3.4. It is also found that the strength of the shock waves, the supersonic jet expansion and the mixing between the primary and secondary fluid flows inside the ejector are strongly affected by the primary pressure. A significant increase in both the shear forces amplitude and the wall friction losses is observed when the boundary layer separates from the ejector wall.

Performance improvement of a double pipe heat exchanger proposed in a small-scale CAES system: An innovative design

Compressed air energy storage (CAES) is a hopeful technology to overcome the intermittency of renewable energy systems and meet the high peak load demand. The objective of this study is to propose a double pipe heat exchanger (DPHX) working with CuO/water nanofluid in order to cool the compressed air before cavern in a small-scale CAES system. A new design of DPHX by considering different internal tube geometry (nine configurations) is proposed. To achieve these targets, a transient model for simulating the technical demeanor of the CAES system is developed. After simulating the behavior of the CAES system, DPHX is modeled by computational fluid dynamics (CFD) to evaluate the outcome of nanofluid as well as geometry design on the DPHX performance. The pressure drop is unchanged for all finned tube at higher Reynolds numbers. The numerical analysis through mathematical modeling of the charging process of the cavern denotes the effect of length and mass flow rate of the secondary fluid in the DPHX. The results illustrate that by enhancing the mass flow of the secondary fluid, the cavern temperature declines. The pressure inside the cavern has a small dependence on its temperature. The cavern pressure is invariant by increasing the secondary fluid flow. For proposed DPHX, the convective heat transfer coefficient increased up to 22% for cold fluid considering tube with four fins (air/nanofluid + finned tube (w = 3.5 mm and H = 1.0 mm)) and compared to the smooth tube. In addition, around 17% enhancement in convective heat transfer coefficient was achieved using tube with eight fins and with air/nanofluid as the working fluid (case with w = 3.5 mm and H = 1.0 mm), compared to tube with four fins. This shows the capability of the proposed finned tube along with the utilization of the nanofluid to increase the heat exchanger performance.

The primary pseudo-shock pattern of steam ejector and its influence on pumping efficiency based on CFD approach

The patterns of primary flow jet core in steam ejector were investigated using computational fluid dynamics (CFD) method. Simulations based on ideal gas model at different operating conditions were performed. The results demonstrate the pattern of primary pseudo-shock flow was affected by the operating conditions obviously when value of operating condition was bigger than that of the critical condition. Compared with the results generated from ideal gas modelling, the primary pseudo-shock flow predicted by wet steam model has a similar pattern and almost the same effective area. But the wet steam model based simulation gives a further downstream choking position and a higher secondary fluid flow velocity at choking position. The above two factors lead to improved entrainment ratio (Em) and increased critical back pressure of steam ejector compared to the results generated from ideal gas modelling. More importantly, the wet steam model predictions show better agreement with the experimental data.

A Review of Airside Heat Transfer Augmentation with Vortex Generators on Heat Transfer Surface

Heat exchanger performance can be improved via the introduction of vortex generators to the airside surface, based on the mechanism that the generated longitudinal vortices can disrupt the boundary layer growth, increase the turbulence intensity and produce secondary fluid flows over the heat transfer surfaces. The key objective of this paper is to provide a critical overview of published works relevant to such heat transfer surfaces. Different types of vortex generator are presented, and key experimental techniques and numerical methodologies are summarized. Flow phenomena associated with vortex generators embedded, attached, punched or mounted on heat transfer surfaces are investigated, and the thermohydraulic performance (heat transfer and pressure drop) of four different heat exchangers (flat plate, finned circular-tube, finned flat-tube and finned oval-tube) with various vortex-generator geometries, is discussed for different operating conditions. Furthermore, the thermohydraulic performance of heat transfer surfaces with recently proposed vortex generators is outlined and suggestions on using vortex generators for airside heat transfer augmentation are presented. In general, the airside heat transfer surface performance can be substantially enhanced by vortex generators, but their impact can also be significantly influenced by many parameters, such as Reynolds number, tube geometry (shape, diameter, pitch, inline/staggered configuration), fin type (plane/wavy/composite, with or without punched holes), and vortex-generator geometry (shape, length, height, pitch, attack angle, aspect ratio, and configuration). The finned flat-tube and finned oval-tube heat exchangers with recently proposed vortex generators usually show better thermohydraulic performance than finned circular tube heat exchangers. Current heat exchanger optimization approaches are usually based on the thermohydraulic performance alone. However, to ensure quick returns on investment, heat exchangers with complex geometries and surface vortex generators, should be optimized using cost-based objective functions that consider the thermohydraulic performance alongside capital cost, running cost of the system as well as safety and compliance with relevant international standards for different applications.

CFD simulation of secondary side fluid flow and heat transfer of the passive residual heat removal heat exchanger

Abstract Passive residual heat removal heat exchanger (PRHR HX) is a key equipment in advanced passive safety pressurized water reactors (PWRs), such as AP1000. Immerged in the In-containment refueling water storage tank (IRWST), the C-shape PRHR HX removes the residual heat using natural convection and boiling heat transfer during postulated accidents. Therefore, the safety operation of the PRHR HX is very important for the nuclear power plant (NPP). In this paper, three dimensional CFD simulation of the secondary side fluid flow and heat transfer of the PRHR HX is performed. The drift flux model is used to simulate the two phase flow phenomenon in the IRWST. The tube region is modeled by the porous media approach. The flow resistance in the tube region is calculated by empirical pressure drop correlation and two phase flow multiplier. The heat transfer rate from the primary side to the secondary side fluid is also evaluated by widely used heat transfer empirical correlations. Additional source terms corresponding to the flow resistance and removed heat in the tube region are added to the momentum and energy equation, respectively. The governing equations are solved by the commercial CFD package FLUENT. Three dimensional distributions of the fluid velocity, temperature and void fraction are obtained. The heat transfer characteristics of the tube bundle is analyzed.

Convective condensation in three enhanced tubes with different surface modifications

An experimental investigation took place in order to determine the convective condensation characteristics of R410A in: two enhanced heat transfer tubes with novel surface modifications; a micro-fin tube and a plain tube. Conditions included: mass flux values ranging from 70 to 330 kg m−2 s−1; saturation temperature is fixed at 313 K; with an inlet quality of 0.9 and outlet quality of 0.2. All tubes evaluated have an outer diameter of 9.52 mm; the heat transfer surface of the 1EHT tube consists of dimples and petal arrays; and the 4EHT tube is produced using trapezoidal dimples in a grid-like arrangement. The experimental results were analyzed using available flow pattern maps. Results indicate that several flow regimes are involved over the range of conditions, including stratified wavy, intermittent and annular. The micro-fin tube exhibits the best enhanced performance with heat transfer coefficients 1.8–2.2 times higher than those of the plain tube; while the 1EHT tube produces a heat transfer coefficient that is 1.4–1.65 times larger than those of the plain tube. Enhancement of the 1EHT tube is achieved by: disruption of the boundary layer; secondary fluid flow generation; fluid mixing caused by a transverse fluid transport; increased fluid turbulence; and increased heat transfer area. When considering enhanced tube frictional pressure drop, the 1EHT tube produces the largest pressure drop and the 4EHT tube is the smallest. Finally, a performance factor comparison was performed in order to evaluate the enhancement effect of the enhanced tubes. This analysis details the superior performance of the 1EHT tube under condensation conditions.

Boiling pressure drop, local heat transfer distribution and critical heat flux in helical coils with R123

The objectives of the present work are to study the influence of curvature on local heat transfer coefficient, two-phase pressure drop and CHF in helical coils. The curvature of coil generates secondary flow of fluid which affects the mechanism inside helical coils. Very less information is available on the influence of curvature on flow boiling system. The present work investigates the characteristics of single phase flow and flow boiling in helical coils. Infra-red thermal imaging technique is used to measure the local wall temperature in the axial direction (along the length of coil) and in the circumferential direction. Experiments are performed with six helical coil test sections made of SS 304 circular tubes having three different tube diameters with two different coil diameters. The tube inner diameters are varying from 5.5 to 9.5 mm with wall thickness of 0.25 mm. The coil diameter to the tube diameter ratio ranges from 14 to 58 and coil pitch is 50 mm. The system pressure is varying from 1.1 bar to 3.6 bar. The analysis is performed for different heated lengths of test section. The experimental data of pressure drop and CHF are compared with available helical coil correlations. In fully developed flow boiling region, the circumferential averaged heat transfer coefficient in helical coils is same as in straight tubes. Correlations are suggested to predict local heat transfer coefficient in single phase and in flow boiling process, two-phase pressure drop and CHF in helical coils.

Transient model of a refrigerant-to-water helically coiled tube-in-tube heat exchanger with corrugated inner tube

A transient model of a refrigerant-to-water helically coiled double tube with corrugations in the inner tube is developed. The refrigerant flows in the annulus, and a secondary fluid (water, glycol/water solution, etc.) flows in the inner tube. The model addresses both the single-phase and the two-phase flows of the refrigerant and the single-phase flow of the secondary fluid. Both the ON/OFF operation of the compressor and of the secondary fluid pump (at compressor shut-down, the pump may be ON or OFF) are considered. To deal with the lack of suitable friction factor and heat transfer correlations, a superposition principle that allows to combine the effects of the curvature of the heat exchanger and of the flutes in the inner tube is adopted. The finite volume method is used to numerically solve the governing equations. The developed model is experimentally validated and achieves reasonable agreement with the experimental data in both steady-state and transient conditions, in both the heating and the cooling modes.

Numerical investigation of flow and heat transfer performances of horizontal spiral-coil pipes

The flow and heat transfer performances of horizontal spiral-coil pipes of circular and elliptical cross-sections are studied. The numerical results are compared with the experimental data, to verify the numerical method. The effects of the inlet water mass flow rate, the structural parameters, the helical pitch and the radius ratio on the heat transfer performances are investigated. Performances of the secondary fluid flow with different radius ratios are also investigated. Numerical results demonstrate that the heat transfer coefficient and the Nusselt number increase with the increase of the water mass flow rate or the helical pitch. The maximum heat transfer coefficient and the maximum Nusselt number are obtained when the radius ratio is equal to 1.00. In addition, the fluid particle moves spirally along the pipe and the velocity changes periodically. The particle flow intensity and the spiral movement frequency decrease significantly with the increase of the radius ratio. Besides, the secondary flow profile in the horizontal spiral-coil pipe contains two oppositely rotating eddies, and the eddy intensity decreases significantly along the pipe owing to the change of curvature. The decreasing tendency of the eddy intensity along the pipe increases with the increase of the radius ratio.

Numerical Analysis of Secondary Flow in the Narrow Gap of Magnetic Fluid Shaft Seal Using a Spectral Finite Difference Method

ABSTRACTIn the article, attention is focused on a secondary flow in the narrow gap of a magnetic fluid shaft seal, and the secondary flow and circumferential speed of the magnetic fluid are numerically evaluated with variation in magnetic fluid plug shape such as angle of the polar head and seal clearance to be varied, using a spectral finite difference method with a new mapped function. Moreover, the maximum pressure drop is calculated in several patterns of equilibrium magnetic fluid plug, and the influence of the centrifugal force is also discussed.

Dynamic Characteristics Analysis on MHTGR Plant’s Secondary Side Fluid Flow Network

Multipe NSSS (Nuclear Steam Supply System) modules use the common feeding-water system to drive the common turbine power generation set. The SSFFN (secondary side fluid flow network) of MHTGR plant has features i.e. strong-coupling and nonlinearity. A wide range of power switching operation will cause unsteady flow, which may destroy the working elements and will be a threat for normal operation. To overcome those problems, a differential-algebraic model and PI controllers are designed for the SSFFN. In MATLAB\SIMULINK environment, a simulation platform is established and used to make a simulation of SSFFN of a MHTGR plant with two NSSS modules, which uses feedwater valves to control the mass flow rate in each module instead of feedwater pump. Results reflect good robustness of controllers.

The Performance of Ground Source Heat Pump System for Domestic Heating under various Ground Water Circulation Conditions

This study is evaluates the performance of a prototype ground source heat pump(GSHP) system under various secondary fluid flow rates. The prototype GSHP has four functions : space heating, cooling, floor heating and domestic water heating. Heating can be operated by two operational modes : water to water and water to air. As a result, the COP of GSHP was increased from 3.43 to 3.84 when the underground circulation flow rate was increased, but ratio of increased in COP was decreased. When the flow rate of the GSHP system including electricity consumption of the ground circulation pump was increased 19.2 LPM to 47.4 LPM, the COP was increased by up to 6.1%, but flow rate was increased 47.4 to 87.7 LPM, the COP was decreased by up to 6.2%. It is suggested that the GSHP with an underground circulation inverter pump saves power consumption in the operation mode, a variable-speed control of ground circulation flow rate based on a compressor Hz have to be selected in order to optimize COP of GSHP system.

High-Frequency Electromagnetic Purification of Silicon

The effect of a high-frequency electromagnetic (EM) field on the removal of nonmetallic inclusions from molten silicon was experimentally investigated. Inclusion separation efficiencies of up to 99 pct were reached. The separation efficiency was independent of the particle concentration in the melt and increased significantly with increases in the frequency, separation time, and coil current. Particles were separated from the silicon matrix and relocated to the top, bottom, and side walls of the crucible due to the effect of three mechanisms: induced secondary fluid flow which carried particles from the bulk of the melt; EM body force which worked in the skin-depth area to trap particles on the side wall; and fluid shear force due to the local acceleration of molten silicon, which promoted the settling of particles to the bottom of the crucible and also carried particles toward the top. Higher coil current enhanced the strength of the magnetic field which enhanced fluid flow, while higher frequency also enhanced the fluid acceleration, and the effect of current was more pronounced leading to better particle separation.

Effect of mean void fraction correlations on a shell-and-tube evaporator dynamic model performance

In this article, the influence of different mean void fraction correlations on a shell-and-tube evaporator dynamic model performance has been evaluated. The model proposed is based on the moving-boundary approach and includes expansion valve modeling. Several transient tests, using R134a as working fluid, have been carried out varying refrigerant mass flow, inlet enthalpy, and secondary fluid flow. Then model performance, using different mean void fractions, is analyzed from the system model outputs (evaporating pressure, refrigerant outlet temperature, and condensing water outlet temperature). The slip ratio expressions selected are a homogenous and momentum flux model and Zivi, Chisholm, and Smith correlations. The results of the comparison between experimental and model predictions depend on the transient characteristics, and there is not a single slip ratio correlation that provides the best performance in all the cases analyzed.

An alternative analysis applied to investigate the ejector performance used in R141b jet-pump refrigeration system

The system performance of R141b jet-pump refrigerator is discussed based on an alternative analysis. It explains the operation of the jet-pump refrigerator operating under actual conditions. A 2 kW cooling of R141b jet-pump refrigerator is designed and constructed to test the ejector performance. With the use of an alternative analysis, point of interest called “critical evaporator temperature” is determined. This point indicates the lowest possible evaporator temperature where the secondary fluid flow is still choked. The critical point varies with the change in operating conditions and with the primary nozzle used. An increase of primary mass flow rate causes the critical point reducing. However, it does not always true. When the primary mass flow rate increases to a certain value, the refrigerator is operated at higher critical evaporator temperature. Thus, the optimum range of the primary mass flow rate for this particular ejector is provided for this present work.

NONAXISYMMETRIC FREE CONVECTION FLOW OVER A ROTATING DISK IN A VISCOELASTIC FLUID WITH MAGNETIC FIELD

The non axisymmetric motion produced by a buoyancy-induced secondary flow of a viscoelastic fluid over an infinite rotating disk in a verticalplane with a magnetic field applied normal to the disk has been studied.The governing Navier Stokes equations and the energy equation admit a self similar solution. The system of ordinary differential equations has been solved numerically using Runge-Kutta Gill subroutine.The turning moment for the viscoelastic fluid is found to be less than that of the Newtonian fluid but the turning moment is increased due to the magnetic parameter. The resultant force due to the buoyancy-induced secondary flow increases with the magnetic parameter but reduces as the viscoelastic parameter increases. The quantity of fluid, which is pumped outwards due to the centrifuging action of the disk, for the viscoelastic fluid is more than that of the Newtonian fluid. The buoyancy-induced secondary flow boundary layer is much thicker than the primary boundary layer thickness. The thermal boundary layer due to the primary flow increases with the magnetic parameter decreases as the viscoelastic parameter increases. The heat transfer increases with the viscoelastic parameter but decreases as the magnetic parameter increases. The effect of the viscoelastic parameter is more pronounced on the secondary flow than on the primary flow.

A comprehensive comparison on vibration and heat transfer of two elastic heat transfer tube bundles

Elastic heat transfer tube bundles are widely used in the field of flow-induced vibration heat transfer enhancement. Two types of mainly used tube bundles, the planar elastic tube bundle and the conical spiral tube bundle were comprehensively compared in the condition of the same shell side diameter. The natural mode characteristics, the effect of fluid-structure interaction, the stress distribution, the comprehensive heat transfer performance and the secondary fluid flow of the two elastic tube bundles were all concluded and compared. The results show that the natural frequency and the critical velocity of vibration buckling of the planar elastic tube bundle are larger than those of the conical spiral tube bundle, while the stress distribution and the comprehensive heat transfer performance of the conical spiral tube bundle are relatively better.