Characterization of exchange flow in vertical pipes of circular and square cross-sections under unstable density gradient

The exchange flows in a buoyancy driven forced flow in vertical pipes of circular and square cross-sections were studied experimentally and the flow structure was examined by proper orthogonal decomposition (POD) analysis. The experiments were carried out in the vertical pipes issuing into a still brine water environment and the flow fields were visualized by the laser-induced fluorescence (LIF) technique. The flow visualization study showed that the critical inflow condition was deviated from the purging boundary due to the influence of hysteresis, and they are shifted to the high Froude numbers in the square pipe compared to the circular one. The POD analysis of the exchange flow indicates that the fluctuating energy is increased in the lower POD modes due to the formation of large-scale structure of the exchange flow. It is found from the analysis that the exchange flow in the low Reynolds number is promoted at the corners of the cross section in the square pipe, while it is randomly distributed in the circular pipe. This corresponds to an increased critical Froude number in the square pipe than that of the circular pipe, which is due to the presence of exchange flow through the corners. With increasing the Reynolds numbers, the flow becomes turbulent and the scale of the exchange flow is decreased and restricted to the near-wall region, while the location of the exchange flow becomes random along the pipe wall irrespective of the cross-sectional shape of the pipes.     

Experimental study of the flow in helical circular pipes: Torsion effect on the flow velocity and turbulence

An objective of the present paper is to experimentally clarify the torsion effect on the flow in helical circular pipes. We have made six helical circular pipes having different pitches and common non-dimensional curvature δ of about 0.1. The torsion parameter β0, which is defined by β0 = τ/（2δ）1/2 with non-dimensional torsion r, are taken to be 0.02, 0.45, 0.69, 1.01, 1.38 and 1.89 covering from small to very large pitch. The velocity distributions and the turbulence of the flow are measured using an X-type hot-wire anemometer in the range of the Reynolds number from 200 to 20000. The results obtained are summarized as follows： The mean secondary flow pattern in a cross section of the pipe changes from an ordinary twin-vortex type as is seen in a curved pipe without torsion （toroidal pipe） to a single vortex type after one of the twin-vortex gradually disappears as β0 increases. The circulation direction of the single vortex is the same as the direction of torsion of the pipe. The mean velocity distribution of the axial flow is similar to that of the toroidal pipe at small β0, but changes its shape as β0 increases, and attains the shape similar to that in a straight circular pipe when ,β0 = 1.89. It is also found that the critical Reynolds number, at which the flow shows a marginal behavior to turbulence, decreases as ,β0 increases for small ,β0, and then increases after taking a minimum at ,β0 ≈ 1.4 as ,β0 increases. The minimum of the critical Reynolds number experimentally obtained is about 400 at ,β0 ≈ 1.4.     

The inward solidification of spheres and circular cylinders

An analytical study is presented of the inward freezing of a sphere or a circular cylinder, initially molten and at the fusion temperature, when the outside surface is suddenly cooled. The treatment assumes, among other things, constant thermal properties and that the parameter β, the ratio of the latent heat to the sensible heat of the substance, is large. Basic series solutions are first derived and these are then followed by a two-region analysis which is needed to accommodate the sharp change in thermal profile just before the centre of the sphere or the cylinder solidifies. Although the theory is strictly asymptotic in nature, results compare well with numerical solutions of the full problem for β = 10 and 20 (cylinder) and β = 10 (sphere).     

Investigation of heat transfer and exergy loss in oscillating circular pipes

In this study, the effect on heat transfer rates, friction factor and exergy loss of oscillating pipe were investigated. Air was used as working fluid. To obtain oscillating motion of the test pipe, the experimental setup was designed. Thermocouple, the temperature control system and the other measurement systems were installed on the oscillating section. For both steady and oscillating flows, the bulk and local wall temperature distribution, pressure drop, inlet and outlet temperatures and frequency were measured. The parameters for this study were chosen as Reynolds number from 5000 to 20,000 at oscillating frequencies with 10 and 20 Hz. The variations of Nusselt number and exergy loss with these parameters were determined and presented graphically. In oscillatory flow, a heat transfer enhancement of up to at constant pumping power was achieved. The exergy loss was decreased with the increase of Reynolds number. The exergy loss affected slightly from the increase of oscillating frequencies.     

Experimental study of flow patterns and improved configurations for pulsating heat pipes

A series of experiments were performed on three types of closed loop pulsating heat pipes(PHPs),intending toinvestigate various kinds of flow patterns,and to develop some improved configurations for the PHPs.Opticalvisualization results indicated that there might exist three flow patterns,i.e.bubble-liquid slug flow,semi-annularflow and annular flow,corresponding to different working conditions.For a given geometry and an adequate fillratio,the PHPs had the self-adjusting characteristic for the flow patterns(from slug flow to semi-annular and thento annular flow)to meet the demands of the increasing heat input.Two special configurations,one with alter-nately varying channel diameter,the other equipped with one section of thicker tube,were found to be advanta-geous in establishing and maintaining reliable circulation of the working fluid.The thermal performance of thePHPs was examined over a range of working conditions.Comparing with the normal PHP with uniform diameter,either of the improved PHPs exhibited higher thermal performance.     

Experimental study on natural convection of MWCNT-water nanofluids in a square enclosure

One of the methods to improve the efficiency of a wide range of energy systems is to enhance the performance of the heat transfer fluids. Nanofluids consisting of multi-walled carbon nanotubes in water base were studied experimentally in a square cavity with differentially heated side walls. The investigation was carried out for particle volume concentrations of 0 to 1% at Ra number 10⁸. The thermal conductivities and viscosities for the nanofluids were experimentally determined. However, it was observed that the available correlations from literature did not agree well with the experimental data. The nondimensional Nusselt number characterised the heat transfer performance. Thermal conductivity was measured for the range of volume concentration 0–1% and maximum enhancement of 6% was found to be at 1%. Viscosity was measured and observed to increase by 58% over the 0–1% particle volume concentration range tested. The experimental results on natural convection yielded a maximum enhancement in heat transfer performance of 45% at volume concentration of 0.1%. This research supports the idea that “for nanofluids with effective thermal conductivity greater than the thermal conductivity of the base fluid, there may exist an optimum concentration which maximizes the heat transfer.”     

垂直地埋管换热器的换热性能

为研究土壤源热泵垂直地埋管换热器的换热特性,对长沙地区一套土壤源热泵系统进行了夏季及冬季工况连续运行的实验,实时采集U型管进出口的水温、流量以及地温等数据。通过对所采集的实验数据进行处理分析,对比了不同工况、不同埋管形式、不同埋深条件下的地埋管换热器进出口温差及单位井深换热量,结果表明,无论是夏季工况还是冬季工况,双U型管的单位井深换热量比单U型管高25%～30%。     

Laminar natural convection in cavities filled with circular and square rods

This work compares heat transfer characteristics across a square cavity partially filled with a fixed amount of conducting solid material. The solid phase is shaped into two different geometries, namely square and cylindrical rods, which are horizontally displaced inside the cavity. Comparisons are obtained by numerically solving a conjugate heat transfer problem that considers both the solid and the fluid space. Governing equations are solved using the finite volume method and the algebraic equation set is relaxed with the SIP procedure. The average Nusselt number at the hot wall, obtained from the cavity with square obstacles and for several Darcy numbers, are compared with those calculated with circular obstacles. When comparing the two geometries considering the same modified Rayleigh number Ra_m, this study shows that the average Nusselt number for cylindrical rods are slightly lower than those for square rods.     

Experimental studies of rotating heat pipes

Thermal characteristics of a rotating heat pipe were measured under steady state at moderate rotational speeds. Copper‐water rotating heat pipe with copper screen mesh wick was fabricated for testing at various heat loads. An experimental test rig with a water‐cooled condenser section was fabricated to study the heat transfer in the rotating heat pipe (RHP) for various heat loads and various rotational speeds ranging from 1000 rpm to 2000 rpm. A heat transfer correlation was developed for the condensing heat transfer coefficient and compared with the experimental results. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20265     

Experimental study of pseudoplastic fluid flows in a square duct of strong curvature

In this paper, laminar and turbulent flows of pseudoplastic fluids (0.1% and 0.2% by weight aqueous solutions of carboxymethylcellulose) in a square duct of strong curvature were measured using an ultrasonic Doppler velocimetry and microphones. Streamwise velocity in cross-sections of the duct and the fluctuating pressure on walls were measured for different flow rates. The velocity contours and their development along the duct were presented and compared with benchmark experiments by Taylor, Whitelaw and Yianneskis (1982) which were for the laminar and turbulent flows of water. The spectra of fluctuating wall pressures were also presented and analyzed. The objective of this paper was to provide a basis for understanding the pseudoplastic fluid flows in curved ducts. The results were also intended for use in the further development of numerical methods and turbulence models for shear-thinning fluids.     

Experimental inward solidification of initially superheated water in a cylinder

The subject of this investigation was the freezing of initially superheated pure water, at small Stefan numbers, contained in a horizontal cylinder. Three experiments were conducted and were compared to an analytical model based upon the heat balance integral method which considered one-dimensional (radial) conductive heat transfer with either zero or a finite amount of initial superheat contained in the water. The equations are solved numerically, employing the Runge-Kutta method for solving the first order governing differential equations. The solution yielded the radius of the phase change boundary as a function of time and the time for the liquid to reach the phase change temperature (0°C) when there is initial superheat. The analytical and experimental solidification time results obtained in this investigation compare very well. As in previous studies, the solidification time was found to be a linear function of Stefan number for zero initial superheat. The analytical results obtained for no initial superheat, though, differ somewhat from the results of some other investigations. Free convection affects appeared to be negligible.     

Experimental study of detonation propagation in a square tube filled with orifice plates

DDT experiments were conducted in a 6000 mm long square cross-section (112 mm × 112 mm) tube with various obstacle configurations with hydrogen-air mixtures and ethylene-air mixtures at ambient pressure (101 kPa) and room temperature (298 K). Square orifice plates with inner side 86.8 mm and 70.8 mm (BR = 0.4 and 0.6) and round orifice plates with inner diameter 80.0 mm (BR = 0.6) were used to assemble the obstacle configurations. The plates were installed at 1, 2 and 3 times the tube inner side. Soot foils were placed between the two orifice plates at the end of the tube for $S=3D$, where $S$ is the obstacle spacing and $D$ is the tube inner side. The DDT limits were determined based on the flame velocity above the isobaric sound speed of the burnt products. The results show that at the DDT limits, the criterion $d_{eff}/\lambda \approx 1$ is not pervasive, i.e., $d_{eff}/\lambda$ decreases with the obstacle spacing increase, in which $d_{eff}$ and $\lambda$ are the effective diameter of the orifice and the detonation cell size. Within the limits, the measured velocity for BR = 0.6 square orifice plates is higher than that for round orifice plates. On the other hand, no obvious difference in the limits can be observed for the BR = 0.6 obstacle configurations. Soot foils provide insights into the detonation propagation mechanism in the orifice plate section. It is shown that hot spots formed via the interactions between the decoupled shock wave and the tube wall can be responsible for the re-initiation of detonation. In addition, overdriven detonations induced by shock focus at the corners, followed by a band of fine cells. For less sensitive mixture and smaller orifice, the re-initiation distance is longer. Near the limits, no cellular structure can be observed, indicating longer cycle period for detonation re-initiation. This also accounts for the significant velocity fluctuation for larger spacing ($S=2D$ and $S=3D$) when the limits are approached.     

Experimental study of flame propagation across flexible obstacles in a square cross-section channel

A comparative study was performed to investigate flame propagation in a square-cross section channel filled with either flexible- or rigid-obstacles with blockage ratio (BR) of 0.429. Experiments were conducted in premixed hydrogen-air mixtures with different equivalence ratios, at initial conditions of 100 kPa and 298 K. High-speed Schlieren photography was used to obtain the detailed flow structure, flame front evolution and the flame tip velocity. Also, pressure transducers were employed to monitor the pressure around the obstacles. Flame propagation across the obstacles was found to be strongly affected by flow contraction induced by obstacles and separated flow pattern downstream of obstacles. Flame propagation with rigid obstacles is mainly governed by the turbulent burning of the fresh gas in the pockets. For the flexible cases, the flow structure is characterized by the shear layer coming off the obstacles leading-corner and the vortex downstream from the obstacles. These special flow structures together provide a flow contraction and constrict flame propagation in the obstacles-free channel, and therefore the flame maintains acceleration. Most notable, the gas flow ahead of the flame purges the flexible obstacle to tilt, yielding an increase in BR, which is correlated with the stronger acceleration as the flame propagates through the obstacles. However, exposing the obstacles to the overpressure for a long period also induces too much deformation. Therefore, the instantaneous BR (BR_real) will also decelerate slightly. Interestingly, BR_real is closely related with the overpressure level.     

Experimental study on freezing of water with supercooled region in a horizontal cylinder

The freezing phenomenon of saturated water with the supercooled region in a horizontal circular cylinder has been studied experimentally by using the holographic real time interferometry technique. From the experiments, it was found that there were three types of freezing patterns. The first is the annular ice layer growing from the cylinder surface at a high cooling rate, the next is the asymmetric ice layer at an intermediate cooling rate, and the last is the instantaneous ice layer growing over the whole region at a low cooling rate. As the water was cooled from room temperature to the subfreezing point passing through the density inversion point, the freezing pattern was largely affected by the inversion phenomenon, which had significantly affected the free convection and was susceptible to influences from the cooling rate. When the cooling rate is high, supercooling energy is released before water is sufficiently mixed by free convection. On the other hand, when the cooling rate is low, there is a lot of time for the water to be mixed by free convection. This seems to be the reason why different ice layer growths occur.     

Experimental study on performance of square tube absorber with phase change material

The intermittent nature of solar radiation has decreased the performance efficiency of solar heaters. Integrating the solar heater with thermal energy storage component could increase its performance effectively. In this article, an investigation on the effect of phase change material (PCM) as the thermal energy storage component on the performance of square aluminum tube was carried out experimentally. In the first phase, the temperature behavior of square aluminum tube with two types of PCM, namely, generic plant-based PCM (A2) and paraffin wax (A3), was compared with square aluminum tube without PCM (A1). In the second phase, the performance of square aluminum tube was investigated with different paraffin wax masses of 38 g (B1), 48 g (B2), and 58 g (B3). Based on the result, the A3 tube configuration performed better than A1 and A2 tube configurations with higher heat gain rate (0.08°C/s) and lower heat discharge rate (−0.04°C/s). The B2 tube configuration was found to have maximum heat gain of 3.73 kJ with higher heat discharge rate as compared with other square tube configurations. The average temperature difference between internal and external surface tube of B2 was lower (4.3°C) leading to higher average temperature difference at ambient temperature of 25.3°C. Instantaneous efficiency of the tube B2 is higher than the B1 and B3 tube configurations by 16% and 26%, respectively. The result suggests that the insertion of paraffin wax inside the square absorber tube improves the temperature response of the absorber in the situation of intermittent solar radiation.     

A numerical study of natural convection in a square enclosure with a circular cylinder at different vertical locations

Numerical calculations are carried out for natural convection induced by a temperature difference between a cold outer square enclosure and a hot inner circular cylinder. A two-dimensional solution for unsteady natural convection is obtained, using the immersed boundary method (IBM) to model an inner circular cylinder based on the finite volume method for different Rayleigh numbers varying over the range of 10³–10⁶. The study goes further to investigate the effect of the inner cylinder location on the heat transfer and fluid flow. The location of the inner circular cylinder is changed vertically along the center-line of square enclosure. The number, size and formation of the cell strongly depend on the Rayleigh number and the position of the inner circular cylinder. The changes in heat transfer quantities have also been presented.     

Experimental investigation of cryogenic oscillating heat pipes

A novel cryogenic heat pipe, oscillating heat pipe (OHP), which consists of an 4 × 18.5 cm evaporator, a 6 × 18.5 cm condenser, and 10 cm length of adiabatic section, has been developed and experimental characterization conducted. Experimental results show that the maximum heat transport capability of the OHP reached 380 W with average temperature difference of 49 °C between the evaporator and condenser when the cryogenic OHP was charged with liquid nitrogen at 48% (v/v) and operated in a horizontal direction. The thermal resistance decreased from 0.256 to 0.112 while the heat load increased from 22.5 to 321.8 W. When the OHP was operated at a steady state and an incremental heat load was added to it, the OHP operation changed from a steady state to an unsteady state until a new steady state was reached. This process can be divided into three regions: (I) unsteady state; (II) transient state; and (III) new steady state. In the steady state, the amplitude of temperature change in the evaporator is smaller than that of the condenser while the temperature response keeps the same frequency both in the evaporator and the condenser. The experimental results also showed that the amplitude of temperature difference between the evaporator and the condenser decreased when the heat load increased.     

Experimental study on heat transfer in square duct with combined twisted-tape and winglet vortex generators

The article presents an experimental investigation on thermal performance enhancement in a constant heat-fluxed square duct fitted with combined twisted-tape and winglet vortex generators. The experiments are carried out for the airflow rate through the tested square duct fitted with both the vortex generators for Reynolds number from 4000 to 30,000. The effect of the combined twisted tape and rectangular winglet inserts on heat transfer and pressure drop presented in terms of respective Nusselt number and friction factor is experimentally investigated. The characteristics of the combined twisted-tape and winglet include two twist ratios (Y = 4 and 5), three winglet- to duct-height ratios, (R_B = 0.1, 0.15 and 0.2), four winglet-pitch to tape-width ratios, (R_P = 2, 2.5, 4 and 5) and a single attack angle of winglet, α = 30°. The experimental results reveal that the Nusselt number and friction factor for the combined twisted-tape and V-winglet increase with increasing R_B but decreasing R_P. The inserted duct at R_B = 0.2, R_P = 2 and Y = 4 provides the highest heat transfer rate and friction factor but the one at R_B = 0.1, R_P = 2 and Y = 4 yields the highest thermal performance. The application of combined vortex-flow devices gives thermal performance around 17% higher than the twisted tape alone.     

Modelling of stratified gas–liquid two-phase flow in horizontal circular pipes

This paper reports numerical and experimental investigation of stratified gas–liquid two-phase flow in horizontal circular pipes. The Reynolds averaged Navier–Stokes equations (RANS) with the k–ω turbulence model for a fully developed stratified gas–liquid two-phase flow are solved by using the finite element method. A smooth interface surface is assumed without considering the effects of the interfacial waves. The continuity of the shear stress across the interface is enforced with the continuity of the velocity being automatically satisfied by the variational formulation. For each given interface position and longitudinal pressure gradient, an inner iteration loop runs to solve the non-linear equations. The Newton–Raphson scheme is used to solve the transcendental equations by an outer iteration to determine the interface position and pressure gradient for a given pair of volumetric flow rates. Favorable comparison of the numerical results with available experimental results indicates that the k–ω model can be applied for the numerical simulation of stratified gas–liquid two-phase flow.