An experimental study into the evaporation heat transfer and flow characteristics of R-134a refrigerant flowing through corrugated tubes

The effects of corrugation pitch on the evaporation of R-134a flowing inside horizontal corrugated tubes are investigated. The test section is a tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water flowing in the annulus. Smooth and corrugated tubes having similar inside diameters are used as the inner tube. Three corrugated tubes having a corrugation depth fixed at 1.5 mm and corrugation pitches of 5.08, 6.35 and 8.46 mm are examined. The data obtained from smooth tube are plotted and compared with the flow pattern map established by Censi et al. (2003) and Zurcher et al. (2002). The effects of average vapour quality, equivalent Reynolds number and corrugation pitch are discussed. The maximum ratios of heat transfer enhancement factor to pressure drop penalty factor (Nu_c/Nu_s)/((ΔP/L)_c/(ΔP/L)_s) is approximately 1.2.     

Heat transfer characteristics of spirally-coiled circular fin-tube heat exchangers operating under frosting conditions

The objective of this study is to investigate the heat transfer characteristics of spirally-coiled circular fin-tube heat exchangers under frosting conditions. The heat transfer rate, pressure drop, frost thickness, and Nusselt number of the heat exchanger were measured and analyzed by varying the fin pitch and number of tube rows under frosting conditions. In addition, the Nusselt number of the spirally-coiled circular fin-tube exchanger was compared with those of flat plate fin-tube heat exchangers with discrete fins. An empirical correlation of the Nusselt number was developed as a function of the Reynolds number, dimensionless fin pitch normalized by the hydraulic diameter, i.e., D_h/F_p, Fourier number, and number of tube rows. The measured Nusselt number was consistent with the predicted value with mean and average deviations of 3.5% and 0.3%, respectively.     

CFD model of ITER CICC. Part VI: Heat and mass transfer between cable region and central channel

Dual-channel cable-in-conduit conductors (CICC) are used in the superconducting magnets for the International Thermonuclear Experimental Reactor (ITER). As the CICC axial/transverse size ratio is typically ∼1000, 1D axial models are customarily used for the CICC, but they require constitutive relations for the transverse fluxes. A novel approach, based on Computational Fluid Dynamics (CFD), was recently proposed by these authors to understand the complex transverse thermal-hydraulic processes in an ITER CICC from first principles. Multidimensional (2D, 3D) Reynolds-Averaged Navier-Stokes models implemented in the commercial CFD code FLUENT were validated against compact heat exchanger and ITER-relevant experimental data, and applied to compute the friction factor and the heat transfer coefficient in fully turbulent spiral rib-roughened pipes, mimicking the central channel of an ITER CICC. That analysis is extended here to the problem of heat and mass transfer through the perforated spiral separating the central channel from the cable bundle region, by combining the previously developed central channel model with a porous medium model for the cable region. The resulting 2D model is used to analyze several key features of the transport processes occurring between the two regions including the relation between transverse mass transfer and transverse pressure drop, the influence of transverse mass transfer on axial pressure drop, and the heat transfer coefficient between central channel and annular cable bundle region.     

Refrigerant R134a vaporisation heat transfer and pressure drop inside a small brazed plate heat exchanger

This paper presents the experimental heat transfer coefficients and pressure drop measured during refrigerant R134a vaporisation inside a small brazed plate heat exchanger (BPHE): the effects of heat flux, refrigerant mass flux, saturation temperature and outlet conditions are investigated. The BPHE tested consists of 10 plates, 72 mm in width and 310 mm in length, which present a macro-scale herringbone corrugation with an inclination angle of 65° and corrugation amplitude of 2 mm.The experimental results are reported in terms of refrigerant side heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show great sensitivity both to heat flux and outlet conditions and weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow.The experimental heat transfer coefficients are also compared with two well-known correlations for nucleate pool boiling and a correlation for frictional pressure drop is proposed.     

Effect of inlet humidity condition on the air-side performance of an inclined brazed aluminum evaporator

The effect of air inlet humidity condition on the air-side heat transfer and pressure drop characteristics for an inclined brazed aluminum heat exchanger has been investigated experimentally. For a heat exchanger with a louver angle of 27°, fin pitch of 2.1 mm and flow depth of 27.9 mm, a series of tests are conducted for the air-side Reynolds numbers of 80–400, with variation of inlet humidity condition. The heat transfer data are obtained for wet condition only and the pressure drop data are measured for both dry and wet conditions. The inlet air temperature and relative humidity range are 12 °C and 60–90%, respectively. The inclination angles (θ) from the vertical position are 0, 14, 45, and 67° clockwise (leeward direction). The inclination angles affect moderately the sensible heat transfer coefficient for wet condition, and the pressure drops for both dry and wet conditions increase systematically with the inclination angle. The heat transfer and pressure drop characteristics under wet condition are not influenced substantially by the air inlet humidity for θ 45°. The effect of the louver directions at the inlet and outlet of the inclined heat exchanger on the performance is also addressed.     

Experimental investigation on flow condensation of methane in a horizontal smooth tube

An experimental investigation on flow visualization of adiabatic and condensation conditions as well as condensation heat transfer coefficient and pressure drop of methane in a horizontal smooth tube was carried out. The tests were conducted at saturation pressure of 2–3.5 MPa with mass flux of 99–255 kg m⁻² s⁻¹ and fluid-to-wall temperature difference of 4.8–20.2 K throughout the vapor quality range. The effects of mass flux, saturation pressure, vapor quality and temperature difference were studied and discussed. In order to expand the range of temperature difference, some condensation heat transfer coefficients of ethane with larger temperature differences (19.7–39.2 K) were also reported in this paper. The experimental data were compared with many well-known correlations of condensation heat transfer coefficient and pressure drop. An improved heat transfer correlation for different flow patterns was proposed and predicted the experimental results well with a mean absolute relative deviation of 6.86%.     

空气侧结构对多元微通道平行流冷凝器传热流动性能的影响

建立了多元微通道平行流冷凝器的稳态分布参数模型,与实验对比验证了模型的正确性。利用所建立的模型,研究了翅片高度、翅片间距、百叶窗开窗间距、百叶窗开窗角度变化对多元微通道平行流冷凝器传热和流动性能的影响。结果表明,随着翅片高度的增大,换热量逐渐增大,空气侧压降逐渐减小;随着翅片间距或者百叶窗开窗间距的增大,换热量和空气侧压降都是逐渐减小;随着百叶窗开窗角度的增大,换热量和空气侧压降都是逐渐增大。     

Pressure drop reduction and heat transfer deterioration of slush nitrogen in triangular and circular pipe flows

Slush fluids such as slush hydrogen and slush nitrogen are characterized by superior properties as functional thermal fluids due to their density and heat of fusion. In addition to allowing efficient hydrogen transport and storage, slush hydrogen can serve as a refrigerant for high-temperature superconducting (HTS) equipment using MgB₂, with the potential for synergistic effects. In this study, pressure drop reduction and heat transfer deterioration experiments were performed on slush nitrogen flowing in a horizontal triangular pipe with sides of 20 mm under the conditions of three different cross-sectional orientations. Experimental conditions consisted of flow velocity (0.3–4.2 m/s), solid fraction (0–25 wt.%), and heat flux (0, 10, and 20 kW/m²). Pressure drop reduction became apparent at flow velocities exceeding about 1.3–1.8 m/s, representing a maximum amount of reduction of 16–19% in comparison with liquid nitrogen, regardless of heating. Heat transfer deterioration was seen at flow velocities of over 1.2–1.8 m/s, for a maximum amount of deterioration of 13–16%. The authors of the current study compared the results for pressure drop reduction and heat transfer deterioration in triangular pipe with those obtained previously for circular and square pipes, clarifying differences in flow and heat transfer properties. Also, a correlation equation was obtained between the slush Reynolds number and the pipe friction factor, which is important in the estimation of pressure drop in unheated triangular pipe. Furthermore, a second correlation equation was derived between the modified slush Reynolds number and the pipe friction factor, enabling the integrated prediction of pressure drop in both unheated triangular and circular pipes.     

Air-side thermal hydraulic performance of multi-louvered fin aluminum heat exchangers

An experimental study on the air-side heat transfer and pressure drop characteristics for multi-louvered fin and flat tube heat exchangers has been performed. For 45 heat exchangers with different louver angles (15–29°), fin pitches (1.0, 1.2, 1.4 mm) and flow depths (16, 20, 24 mm), a series of tests were conducted for the air-side Reynolds numbers of 100–600, at a constant tube-side water flow rate of 0.32 m³/h. The inlet temperatures of the air and water for heat exchangers were 21 and 45°C, respectively. The air-side thermal performance data were analyzed using effectiveness-NTU method for cross-flow heat exchanger with both fluid unmixed conditions. The heat transfer coefficient and pressure drop data for heat exchangers with different geometrical configurations were reported in terms of Colburn j-factor and Fanning friction factor f, as functions of Reynolds number based on louver pitch. The general correlations for j and f factors are developed and compared to other correlations. The f correlation indicates that the flow depth is one of the important parameters for the pressure drop.     

同轴套管换热器的优势初探

对螺旋槽纹管进行大量的仿真计算和试验研究,分析内部的多维旋转扰动流动方式及其对强化传热的影响;初步探讨槽深和螺距与换（传）热系数及管内水阻力的关系,为换热器内部结构的优化提供参考依据。     

Computational study on effects of rib height and thickness on heat transfer enhancement in a rib roughened square channel

A computational study was carried out for the heat transfer augmentation in a three-dimensional square channel fitted with different types of ribs. The standard k–ε model and its two variants (RNG and realizable) were used for turbulence modeling. The predictions were compared with available experimental and computational results. Three rib configurations were used in the present study: 90° continuous attached ribs, 60° V-shaped broken attached thick and thin ribs. It was observed that the maximum heat transfer occurs at the normalized rib spacing (p/e) = 10 in the case of 90° attached ribs. The effects of the blockage ratio and rib thickness were investigated for 60° V-shaped broken ribs with Re = 10,000–30,000 and p/e = 10. It was observed that the average Nusselt number decreases with an increase in the Reynolds number for almost all configurations studied in the present study. For the 60° V-shaped broken ribs, increasing the blockage ratio had an adverse effect on the heat transfer. It was also observed that thin ribs perform better than thick ribs.     

Heat transfer enhancement by winglet-type vortex generator arrays in compact plain-fin-and-tube heat exchangers

The potential of winglet type vortex generator (VG) arrays for air-side heat transfer enhancement is experimentally evaluated by full-scale wind-tunnel testing of a compact plain-fin-and-tube heat exchanger. The effectiveness of a 3VG alternate-tube inline array of vortex generators is compared to a single-row vortex generator design and the baseline configuration. The winglets are placed in a common-flow-up orientation for improved tube wake management. The overall heat transfer and pressure drop performance are assessed under dry-surface conditions over a Reynolds number range based on hydraulic diameter of 220 ≤ Re ≤ 960. It is found that the air-side heat transfer coefficient increases from 16.5% to 44% for the single-row winglet arrangement with an increase in pressure drop of less than 12%. For the three-row vortex generator array, the enhancement in heat transfer coefficient increases with Reynolds number from 29.9% to 68.8% with a pressure drop penalty from 26% at Re = 960 to 87.5% at Re = 220. The results indicate that vortex generator arrays can significantly enhance the performance of fin-tube heat exchangers with flow depths and fin densities typical to those used in air-cooling and refrigeration applications.     

Spontaneous twist and intrinsic instabilities of pristine graphene nanoribbons

In pristine graphene ribbons, disruption of the aromatic bond network results in depopulation of covalent orbitals and tends to elongate the edge, with an effective force of f _e ～ 2 eV/Å (larger for armchair edges than for zigzag edges, according to calculations). This force can have quite striking macroscopic manifestations in the case of narrow ribbons, as it favors their spontaneous twisting, resulting in the parallel edges forming a double helix, resembling DNA, with a pitch t of about 15–20 lattice parameters. Through atomistic simulations, we investigate how the torsion τ ～ 1/λ _t decreases with the width of the ribbon, and observe its bifurcation: the twist of wider ribbons abruptly vanishes and instead the corrugation localizes near the edges. The length-scale (λ _e) of the emerging sinusoidal “frill” at the edge is fully determined by the intrinsic parameters of graphene, namely its bending stiffness D=1.5 eV and the edge force f _e with λ _e ～D/f _e. Analysis reveals other warping configurations and suggests their sensitivity to the chemical passivation of the edges, leading to possible applications in sensors.      

The study of entropy generation during flow boiling in a micro-fin tube

Increasing in the heat transfer rate in flow boiling is a common and key issue for engineers. Generally, the heat transfer coefficient augmentation methods are divided into two main categories (active and passive methods). In passive methods the increase in heat transfer rate causes the increase in pressure drop. In order to evaluate the contribution of heat transfer and pressure drop mechanisms, the entropy generation analysis is used. In this paper, the entropy generation in micro-fin tube is investigated under flow boiling condition. The effect of different geometrical parameters and flow conditions is discussed on pressure drop contribution and heat transfer one in entropy generation, irreversibility distribution ratio (IDR) and Bejan number (Be). The frictional pressure drop and heat transfer coefficient in the micro-fin tube and the helically coiled one are compared as two enhancements passive heat transfer methods with the smooth straight tube in the literatures. Therefore, by introducing entropy generation number (N_s), the favorable geometry between the micro-fin tube and the helically coiled one with respect to the smooth straight tube is recognizable at equivalent boundary conditions.     

Heat transfer in cryogenic liquids under pressure

The dependence of the heat transfer on pressure is studied for a horizontal plane surface in liquid helium, liquid hydrogen, and liquid nitrogen in the range 0.03 < p/p_c < 0.9: a maximum of the maximum heat flux density q?_m,m was found at ～0.35 p_c. For helium, additionally, the dependence on the inclination of the surface against the horizontal was determined.A general representation for the dependence of the reduced maximum heat flux density q?_m/q_m,m on the reduced pressure p/p_c is given which also describes adequately well the results obtained with other cryogenic liquids.     

Heat transfer and hydrodynamics in cooling channels of superconducting devices

The results of experimental studies of heat transfer, critical heat flux and pressure drop in forced flow of cryogens are presented. Analysis of different flow and heat transfer conditions was carried out using the results of present studies and the data of other authors. Significant effects of operating conditions on hydraulic and temperature characteristics of cooling channels in superconducting devices were demonstrated. The influence of a heating transient on heat transfer to liquid helium was examined. Correlations were developed which are in close agreement with available heat transfer and pressure drop data for forced flow of cryogens.     

Conjugate heat transfer in solar collector panels with internal longitudinal corrugated fins—Part I: Overall results

A computational study is conducted to examine the fully developed laminar flow and heat transfer characteristics in solar collector panels with internal, longitudinal, corrugated fins. The fins are integrally attached to the upper and lower panel walls. The objective of the study is to determine the effects of varying the fin pitch (or fin angle), the fin thickness, the ratio of the thermal conductivity of the panel walls and the fin to that of the fluid, and the thermal boundary condition on the panel heat transfer and pressure drop. The solutions of the momentum and energy equations are obtained by using a control-volume-based finite difference algorithm. The results of the study are also applicable to the design of internally-finned channels in compact heat exchangers. The overall panel heat transfer increases when the fin pitch (or the fin angle) is decreased, when the fin thickness is increased, and when the thermal conductivity ratio is increased. The streamwise pressure drop increases with decreasing fin pitch (or fin angle) and increasing fin thickness. For a fixed fin thickness, the selection of a small fin pitch (or fin angle) over the range studied results in a higher heat transfer enhancement per unit pumping power.     

Numerical Study and Optimization of Hydrothermal Characteristics of Mn–Zn Ferrite Nanofluid Within Annulus in the Presence of Magnetic Field

Flow and heat transfer characteristics of water–MnZnFe₂O₄ magnetic nanofluid through an annulus were evaluated under the effect of non-uniform magnetic field using the two-phase Euler–Lagrange method. The effects of concentration, size of particles and magnitude of magnetic field gradient were investigated. The concentration distribution was found to be non-uniform, with its value lower near the walls. Velocity profile becomes flatter at the cross section of the annulus by applying the magnetic field. Increasing particle size, concentration and magnitude of the magnetic field gradient enhance the convective heat transfer coefficient. The effect of increasing magnitude of the magnetic field gradient on heat transfer and pressure drop is more significant for larger particles. Models of convective heat transfer coefficient and pressure drop were obtained in terms of the effective parameters using neural network. Meanwhile, optimization was implemented via genetic algorithm coupled with compromise programming technique in order to achieve the conditions with maximum heat transfer and minimum pressure drop. Based on the results obtained from optimization, application of the magnetic field is only recommended when heat transfer is considered to be more important than pressure drop.     

Pressure drop and heat transfer during two-phase flow vaporization of propane in horizontal smooth minichannels

This study examined the two-phase flow boiling pressure drop and heat transfer for propane, as a long term alternative refrigerant, in horizontal minichannels. The pressure drop and local heat transfer coefficients were obtained for heat fluxes ranging from 5–20 kW m^?2, mass fluxes ranging from 50–400 kg m^?2 s^?1, saturation temperatures of 10, 5 and 0 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and lengths of 1000 mm and 2000 mm, respectively. The present study showed the effect of mass flux, heat flux, inner tube diameter and saturation temperature on pressure drop and heat transfer coefficient. The experimental results were compared against several existing pressure drop and heat transfer coefficient prediction methods. Because the study on evaporation with propane in minichannels was limited, new correlations of pressure drop and boiling heat transfer coefficient were developed in this present study.     

Effect of louver angle on performance of heat exchanger with serpentine fins and flat tubes in frosting: Importance of experiments in periodic frosting

This paper presents the air-side pressure drop and overall heat transfer coefficient characteristics for serpentine-louvered-fin, microchannel heat exchanger in periodic frosting. It focuses on quantification of the effects of louver angle on heat transfer and pressure drop and on defrost and refrost times. Nine heat exchangers differing in louver angle and fin pitch (i.e. louver angle 15° to 39° and fin pitch 12 to 18 fpi) are studied. The face velocity was 3.5 m s⁻¹ and inlet air relative humidity of 70% and 80%.The major finding of this paper is seen in the proof that it is necessary to perform experiments in several consecutive cycles to be able to understand the situation in operation of the heat exchangers and that geometry that is the best in first frosting test does not stay the best in real operation where multiple consecutive frosting occurs.