Measurement and prediction of heat transfer coefficient on ammonia flow boiling in a microfin plate evaporator

Thermal characteristics of ammonia flow boiling in a microfin plate evaporator are experimentally investigated. Titanium microfin heat transfer surface is manufactured to enhance boiling heat transfer. Longitudinally- and laterally-microfined surfaces are used and those performances are compared. Heat transfer coefficient of microfin plate evaporator is also compared with that of plain-surface plate evaporator. The effects of mass flux, heat flux, channel height, and saturation pressure on heat transfer coefficient are presented and discussed. The experiments are conducted for the range of mass flux (5 and 7.5 kg m⁻² s⁻¹), heat flux (10, 15, and 20 kW m⁻²), channel height (1, 2, and 5 mm), and saturation pressure (0.7 and 0.9 MPa). Heat transfer coefficient is compared with that predicted by available empirical correlations proposed by other researchers. Modified correlations using Lockhart-Martinelli parameter to predict heat transfer coefficient are developed and they cover more than 87% of the experimental data.     

A new model for flow boiling heat transfer coefficient inside horizontal tubes with twisted-tape inserts

This paper presents a new method to predict the heat transfer coefficient during flow boiling inside horizontal tubes containing twisted tape inserts. The method was developed based on the database presented by Kanizawa et al. This database comprises flow boiling results for horizontal tubes with internal diameters of 12.7 and 15.9 mm, twisted-tape ratios of 3, 4, 9 and 14, mass velocities ranging from 75 to 200 kg m⁻² s⁻¹, heat fluxes of 5 and 10 kW m⁻² and saturation temperatures of 5 and 15 °C. The method is flow-pattern based and considers flow boiling, dryout and mist flow regions. The predictive method also takes into account the physical picture of the swirl flow phenomenon by considering swirl flow effects promoted by the twisted tape insert. The proposed method provides satisfactory predictions and captures the main heat transfer trends of the data of Kanizawa et al. and also of independent data from literature.     

A note on the Chen correlation of saturated flow boiling heat transfer

The Chen (Ind. Eng. Chem. Process Des. Dev., 1966, 5(3): 322–329) correlation of saturated flow boiling heat transfer is one of the most influential flow boiling heat transfer correlations. It adopted the additive concept and incorporated the Reynolds number factor F and the suppression factor S. Chen presented F and S as the best-fit curves without any parametric equation. However, the parametric equations of F and S were widely used in citing the Chen work, among which some were not accurate, and some had typos in the original sources and then used by others. The objective of this paper is to point out the incorrect and inaccurate equations in the literature. For this purpose, the existing expressions of F and S in the available literature are presented and compared with the Chen best-fit curves. The work provides a reference for the correct use of the Chen correlation.     

Correction of logarithmic mean temperature difference in a compact brazed plate evaporator assuming heat flux governed flow boiling heat transfer coefficient

The heat transfer in heat exchangers is commonly calculated using the concept of Logarithmic Mean Temperature Difference (LMTD). As is well known this approach is only valid for counter-current and co-current heat exchanger configurations. For other configurations, corrections for the deviation from pure counter-current are introduced. From any standard text book in heat transfer it may be found that the LMTD approach may also be used if condensation and evaporation occurs in the heat exchanger. The purpose of the present paper is to investigate if the LMTD approach can be used in a compact brazed plate evaporator. It will be shown through integration of the governing equations that the LMTD approach indeed may be used for practical cases, even though deviations occur at small logarithmic mean temperature differences. The article presents suggestions on the correction factor (F) needed under some simplified assumptions in a compact brazed plate heat exchanger operating as an evaporator for heat pump and refrigeration applications.     

Effect of inlet configuration on the refrigerant distribution in a parallel flow minichannel heat exchanger

The refrigerant R-134a flow distribution was experimentally studied for a round header/ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet configurations (parallel, normal, vertical) were investigated. Tests were conducted with downward flow for mass flux from 70 to 130 kg m⁻² s⁻¹ and quality from 0.2 to 0.6. Tubes were flush-mounted in the test section, with no protrusion into the header. It is shown that normal and vertical inlet yielded similar flow distribution. As mass flux or quality increased, however, better results were obtained for normal inlet configuration. The flow distribution was worst for the parallel inlet configuration. Possible explanation is provided based on flow visualization results. Correlations were developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream.     

Experiments on dry-out during flow boiling in a round minichannel

Temperature measurements during flow boiling of R134a in a 0.96 mm single circular channel are reported in order to provide a criterion for the determination of the critical conditions in the channel. The flow boiling heat transfer is obtained by using a secondary fluid; the wall temperature displays larger fluctuations in the zone where dryout occurs. These temperature fluctuations in the wall denote the presence of a liquid film drying up at the wall with some kind of an oscillating process. These temperature fluctuations never appear during condensation tests, neither are present during flow boiling at low vapour qualities. The fluctuations also disappear in the post-critical condition zone. Experimental values of dryout quality measured with the above method are reported in this paper at mass velocity ranging between 300 and 600 kg m^?2s^?1. In the practical applications of flow boiling, the dryout quality is a key parameter in the two-phase systems for cooling of devices, both for ground and microgravity applications. The test conditions reported here refer to relatively high mass velocities, and are obtained at earth gravity. Nevertheless, since the critical heat flux differences between the two gravity environments decrease with increasing velocity, the present data may also be used for inertia dominated systems at low g.     

Comprehensive correlation for dispersed flow film boiling heat transfer in mini/macro tubes

A general correlation for heat transfer during dispersed flow film boiling is presented which is applicable to horizontal and vertical tubes. It is based on the two-step model. It has been verified with data for 10 fluids in horizontal and vertical upflow. The fluids include refrigerants, hydrocarbons, cryogens, CO₂ and water. The range of data included diameters from 0.98 to 25.0 mm, reduced pressures from 0.0046 to 0.99, mass flux from 3.7 to 5176 kg m⁻² s⁻¹, and qualities from 0.1 to 2.96. The 1481 data points from 38 sources are predicted with mean absolute deviation of 19.4%. Several other correlations are also compared to the same data and found to have much larger deviations.     

Local boiling heat transfer characteristics of ammonia/water binary mixture in a vertical plate evaporator

The next-generation energy production systems are expected to be based on ocean thermal energy conversion (OTEC) and discharged thermal energy conversion (DTEC). These systems use a plate-type evaporator and ammonia or an ammonia/water mixture as a working fluid. It is important to clarify heat transfer characteristics for designing efficient power generation systems. Measurements of local boiling heat transfer coefficients and visualizations were performed for an ammonia/water mixture (z = 0.9) on a vertical flat plate heat exchanger at a range of mass fluxes (7.5-15 kg m⁻² s⁻¹), heat fluxes (15-23 kW m⁻²), and pressures (0.7-0.9 MPa). The results show that in the case of an ammonia/water mixture, the local heat transfer coefficients increase with an increase in the vapor quality and mass flux and decrease with an increase in the heat flux. The influence of the flow pattern on the local heat transfer coefficient is also observed.     

MHD flow between two parallel plates with heat transfer

Summary In the present paper, the steady flow of an electrically conducting, viscous, incompressible fluid bounded by two parallel infinite insulated horizontal plates and the heat transfe through it are studied. The upper plate is given a constant velocity while the lower plate is kept stationary. The viscosity of the fluid is assumed to vary with temperature. The effect of an external uniform magnetic field as well as the action of an inflow perpendicular to the plates together with the influence of the pressure gradient on the flow and temperature distributions are reported. A numerical solution for the governing non-linear ordinary differential equations is developed.     

Specifics of boiling and condensation in upward flow in minichannel systems

The results of experimental and numerical studies focused on determining the mechanism of heat transfer during boiling and condensation in a single-row system of minichannels in upward flow conditions at a mass flux of 30 and 50 kg/(m² s) are presented. Refrigerant R21, which models cryogenic liquids at low temperatures, was used as the working liquid. The determining influence of self-organization of the flow under the influence of capillary forces on the processes of heat transfer during a phase transition in the system of minichannels at low mass and heat fluxes was revealed.     

The flow of a viscoelastic fluid between two parallel plates with heat transfer

The problem of the flow of a third grade fluid between two parallel plates with heat transfer is studied. We prove that von Kármán type solutions are not admissible for a general third grade fluid, but it may be experienced by a particular subclass which we put in evidence. The existence, the uniqueness and the dependence on the little parameters a = RePr of the solution of the heat transfer problem are then analysed. Some numerical experiments concerning the first two approximations of the attached Taylor expansion of the solution of this problem are represented.     

制冷剂-润滑油互溶混合物系流动沸腾特性测试装置的研制

在对已有相关测试装置设计技术的利弊分析基础上,提出了一种测试制冷剂-润滑油互溶混合流体管内流动沸腾特性的装置,其特征是通过与制冷剂流动沸腾换热测量回路并联接入一开式的润滑油回路,从而实现灵活控制和调节实验段内润滑油量的目的.通过在压缩机出口串连接入3个油分离器进行三级油分,从而实现压缩机出口的润滑油零排放.该装置具有连续在线注油、达到稳态时间短、操作灵活、控制简单等优点.     

Experimental investigations on heat transfer characteristics of pulsating single-phase liquid flow and two-phase Taylor bubble flow through a minichannel

Experimental investigations are reported for pulsating Taylor bubble (PTB) flow through a 2.12 mm horizontal circular minichannel. Air and water are used as working fluids. A T-junction is used to generate Taylor bubble flow in a minichannel. The superficial gas velocity (U _SG ) is kept as 0.0472 m/s. The superficial liquid velocity (U _SL ) is kept as 0.0472 and 0.0708 m/s. The pulsating liquid flow is generated by developing a pulse generator circuit. The investigations are carried out for various pulsating flow frequencies of 0 Hz (continuous flow), 0.1, 0.25, 0.5, 1 and 2 Hz, which correspond to Womersley number (W _o ) 0, 0.84, 1.39, 1.88, 2.65 and 3.75, respectively. Heat transfer enhancement is found to be negligible (less than 1%) for pulsating laminar liquid flow through the minichannel. On the contrary, heat transfer is observed to decrease by 35% for PTB flow compared with continuous Taylor bubble (CTB) flow for imposed frequency of pulsation up to 1 Hz.     

Forced flow boiling heat transfer of liquid hydrogen for superconductor cooling

Heat transfer from inner side of a heated vertical pipe to liquid hydrogen flowing upward was first measured at the pressure of 0.7 MPa for wide ranges of flow rates and liquid temperatures. The heat transfer coefficients in non-boiling regime for each flow velocity were well in agreement with the Dittus–Boelter equation. The heat fluxes at the inception of boiling and the departure from nucleate boiling (DNB) heat fluxes are higher for higher flow velocity and subcooling. It was found that the trend of dependence of the DNB heat flux on flow velocity was expressed by the correlation derived by Hata et al. based on their data for subcooled flow boiling of water, although it has different propensity to subcooling.     

A mathematical model of heat transfer for forced-convective film boiling

A mathematical model of film boiling with forced convection of the liquid is discussed. A solution is obtained on a computer and is represented in the form of nomographs and approximation relations. The computed results are compared with experimental data.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 5, pp. 822–827, November, 1977.     

Analogies between heat and momentum transfer in a forced liquid flow with surface boiling in a channel

Consideration is made of the characteristics of heat- and momentum-transfer phenomena in a forced liquid flow under conditions of surface boiling in a channel.     

Convective heat transfer in a metastable liquid with delayed boiling

An analysis is made of the steady-state heat transfer in a large volume of diethyl ether, npentane, and benzene with a thin wire immersed. A 90°C superheat was produced at the wire surface, while the bulk liquid remained at saturation temperature.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 22, No. 4, pp. 614–617, April, 1972.     

Performance comparison of an air/water heat pump using a minichannel coil as evaporator in replacement of a fin-and-tube heat exchanger

This paper reports the results of an experimental and numerical study of the use of a minichannel coil and a fin-and-tube coil in an air/water heat pump working as evaporator when the system is working in heating mode. The experimental installation developed to test the heat pump unit is briefly described and then the experimental results are presented. Alternatively, the heat pump is modelled using a refrigeration system design program. The results obtained using this model are compared to experimental results allowing the validation of the model. Once the model is validated, it is used to numerically determine the distribution of refrigeration charge and other interesting parameters not experimentally measured. Finally, an analysis of the numerically obtained results is presented in order to study the influence of several operating conditions on refrigerant charge, system COP and heating power for both types of evaporators tested.     

Review of correlations of flow boiling heat transfer coefficients for carbon dioxide

Carbon dioxide (CO₂) has quite different flow boiling heat transfer characteristics from conventional refrigerants due to its much higher reduced pressures that make its thermodynamic and transport properties very different. There were some studies evaluating the correlations of flow boiling heat transfer coefficient for CO₂. However, either the number of correlations covered or the number of data used was limited, resulting in inconsistent conclusions. This work presents a comparative review of existing correlations for flow boiling heat transfer coefficient of CO₂. There are 34 correlations analyzed and evaluated using 2956 experimental data points of CO₂ flow boiling heat transfer from 10 independent laboratories. The Fang (2013) correlation performs best with a mean absolute deviation of 15.5%. The evaluation analysis sets a channel transition criterion for flow boiling heat transfer of CO₂. Several topics worthy of attention for future studies are identified.     

Enhancing flow boiling heat transfer in microchannels for thermal management with monolithically-integrated silicon nanowires

Thermal management has become a critical issue for high heat flux electronics and energy systems. Integrated two-phase microchannel liquid-cooling technology has been envisioned as a promising solution, but with great challenges in flow instability. In this work, silicon nanowires were synthesized in situ in parallel silicon microchannel arrays for the first time to suppress the flow instability and to augment flow boiling heat transfer. Significant enhancement in flow boiling heat transfer performance was demonstrated for the nanowire-coated microchannel heat sink, such as an early onset of nucleate boiling, a delayed onset of flow oscillation, suppressed oscillating amplitudes of temperature and pressure drop, and an increased heat transfer coefficient.