Flow boiling in small diameter channels

Many studies have been carried out on two-phase flow heat transfer in channels with hydraulic diameter bigger than 6 mm, but relatively little work has been done for small diameter channels in the meso and compact range (diameter from 0.1 to 3 mm). The use of exchangers with small channels in refrigeration units, which are very numerous besides, could bring a significant reduction of the internal volume of the exchanger, and therefore diminish the refrigerant charge of the whole refrigerating system. One can imagine the interest to widen knowledge on the flow and the heat transfer in small-diameter tubes. This paper examine the thermal behavior of refrigerants boiling in small pipes. The correlations available for in-tube evaporation heat transfer coefficient are discussed and evaluated, when possible, and new research areas of relevance than can contribute to expand the use of small-diameter channels evaporators in refrigeration units are suggested.     

Flow boiling of R32 inside a brazed plate heat exchanger

This paper presents measurements of heat transfer coefficient obtained during flow boiling of R32 inside a brazed plate heat exchanger (BPHE). Although R32 is known as a very interesting refrigerant for its thermodynamic and thermophysical properties, very limited flow boiling data are published in the open literature for R32 working in brazed plate heat exchangers.The present experimental data are measured to investigate the effect of refrigerant heat flux, mass velocity, inlet vapor quality and superheating at the outlet. The saturation temperature is kept constant at around 5 °C, which is a usual temperature level for evaporation in liquid coolers. As a significant result, differently from other studies on flow boiling with HFC refrigerants, mass flux is found to be very important, meaning a high contribution of the convective term on the heat transfer coefficient.The present data are also analyzed to assess available correlations for flow boiling inside BPHEs, in order to provide useful information on the accuracy of predicting methods that can be used for evaporators with R32.     

Flow boiling of ammonia and hydrocarbons: A state-of-the-art review

A comprehensive review of flow boiling heat transfer, two-phase pressure drops and flow patterns of ammonia and hydrocarbons applied in air-conditioning, refrigeration and heat pump systems is presented in this paper. First, experimental studies of flow boiling of ammonia and hydrocarbons are addressed. Then, the prediction methods for flow boiling heat transfer, two-phase pressure drops and flow patterns are described. Next, comparisons of four flow boiling heat transfer and four two-phase pressure drop methods to the experimental data in smooth tubes derived from the available studies are presented. In addition, comparison of flow patterns to a flow map is presented. Based on the comparisons and analysis, recommendations on these methods are given. Furthermore, research needs on flow boiling and two-phase flow of ammonia and hydrocarbons have been identified. It is suggested that more experimental data be obtained through well conducted experiments and new prediction methods or modified ones based on the available methods be made for ammonia and hydrocarbons. In addition, the effect of oil on ammonia and hydrocarbon flow boiling and two-phase flow should be studied in order to have conclusive evidence of its effect.     

Heat transfer and pressure drop for boiling nitrogen flowing in a horizontal tube: 1. Saturated flow boiling

Forced convection boiling of liquid nitrogen in a smooth horizontal copper tube with 14 mm id has been studied experimentally. The measured local heat transfer coefficients in nucleate boiling depend on the heat flux as well as on the mass flow rate. Furthermore, the influence of the vapour quality cannot be neglected.Our own experimental heat transfer data were correlated by an empirical equation. Mass flow rate, pressure, and diameter dependence of para-hydrogen data of other authors can also be correlated with this equation. A relationship for the critical heat flux is also given.     

Experimental heat transfer coefficients of pool boiling and spray evaporation of ammonia on a horizontal plain tube

We have determined experimentally both spray evaporation and pool boiling heat transfer coefficients, using ammonia and titanium plain tubes, to compare both processes and to contribute to the existing database. In the paper we have detailed the experimental equipment used and the methodology followed. We have also exposed the data analysis process followed to obtain spray evaporation or pool boiling heat transfer coefficients from the experimental data measured and we present and discuss the results obtained.Spray evaporation heat transfer coefficients depend on heat flux and on the refrigerant film flow rate. At the high heat flux range, spray evaporation heat transfer coefficients decrease with decreasing film flow rates, suggesting the existence of dry patches, which we confirmed visually. Spray evaporation is particularly beneficial at the low heat flux range studied. Under certain conditions of high heat flux, pool boiling outperforms spray evaporation due to the heat transfer coefficient deterioration caused by dry patches.     

Flow boiling heat transfer coefficients at cryogenic temperatures for multi-component refrigerant mixtures of nitrogen–hydrocarbons

The recuperative heat exchanger governs the overall performance of the mixed refrigerant Joule–Thomson cryocooler. In these heat exchangers, the non-azeotropic refrigerant mixture of nitrogen–hydrocarbons undergoes boiling and condensation simultaneously at cryogenic temperature. Hence, the design of such heat exchanger is crucial. However, due to lack of empirical correlations to predict two-phase heat transfer coefficients of multi-component mixtures at low temperature, the design of such heat exchanger is difficult.The present study aims to assess the existing methods for prediction of flow boiling heat transfer coefficients. Many correlations are evaluated against available experimental data of flow boiling of refrigerant mixtures. Silver-Bell-Ghaly correlation and Granryd correlation are found to be more suitable to estimate local heat transfer coefficients. A modified Granryd correlation is recommended for further use.     

An experimental investigation of saturated flow boiling heat transfer and pressure drop in square microchannels

In this study, saturated flow boiling characteristics of deionized water in parallel microchannels are investigated experimentally. The silicone microchannel heat sink consists of 29 parallel square microchannels having hydraulic diameters of 150 µm. Experiments have been conducted for four different values of the mass flux consisting of 51, 64.5, 78 and 92.6 kg/m²s and heat flux values from 59.3 to 84.1 kW/m². Inlet temperature of deionized water is kept at 50 ± 1 °C. Heat transfer and pressure drop are examined for varying values of the governing parameters. Simultaneous high-speed video images have been taken as well as temperature and pressure measurements. The flow visualization results lead to key findings for flow boiling instabilities and underlying physical mechanisms of heat transfer in microchannels. Quasi-periodical rewetting and drying, rapid bubble growth and elongation toward both upstream and downstream of the channels and reverse flow are observed in parallel microchannels.     

Heat transfer in nucleate boiling of different low boiling substances

Heat transfer coefficients for nucleate boiling of methane, ethane, ethylene, argon and carbon dioxide were determined using an apparatus for the precise investigation of pool boiling heat transfer in the low temperature range. The apparatus used a horizontal cylinder as the heating element. The influence of the thermophysical properties of the boiling liquid was established by comparing the absolute values of the heat transfer coefficients in a normalized boiling state, i.e. a saturation pressure equal to 10% of the critical pressure and a heat flux density equal to 2 × 10⁴ W m⁻². By including the results for a number of higher boiling liquids, which were investigated previously under similar experimental conditions, and using literature data for three very low boiling liquids, an empirical correlation is established which allows an approximate prediction of the absolute value of the heat transfer coefficient at nucleate boiling for substances of different molecular structure.     

微细通道换热器的研究进展

微细通道换热器不仅体积换热系数大、换热效率高,而且具有优良的耐压性能、较强的抗腐蚀性、紧凑的结构及相对低廉的价格,已成为相关领域的一个研究热点。本文从微细通道换热器内流体流动摩擦特性,流体的单相对流换热、凝结换热、沸腾换热,临界热流密度,微细通道换热器结构的优化,结霜问题及其在制冷空调系统的应用等方面,对微细通道换热器进行较为详细的综述,以期为相关的研究领域提供有价值的参考。     

Cryogenic liquids forced boiling heat transfer in a rotating channel

Forced flow boiling heat transfer to nitrogen and helium in a channel rotating around a parallel axis is investigated. It is found that the heat transfer coefficient and critical heat flux vary greatly along the channel perimeter. Generalized equations have been suggested on the basis of experimental data.     

Simulation of subcooled flow boiling in a micro-channel

Bubble behavior was simulated to analyze the mechanism of subcooled boiling in a micro-channel. Bubble growth, condensation, and collapse in subcooled boiling, as well as the function of the degree of subcooling, lift-off diameter, heat flux, and mass flux are discussed. The influence of surface tension on interfacial heat transfer is likewise presented. In the calculated results, both onset of nucleate boiling and boiling heat transfer are influenced by micro-channel size. The simulated flow pattern corresponds with some experimental results. Findings on the boiling heat transfer from the simulation were compared with experiments.     

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.     

High heat flux transport by microbubble emission boiling

In highly subcooled flow boiling, coalescing bubbles on the heating surface collapse to many microbubbles in the beginning of transition boiling and the heat flux increases higher than the ordinary critical heat flux. This phenomenon is called Microbubble Emission Boiling, MEB. It is generated in subcooled flow boiling and the maximum heat flux reaches about 1 kW/cm²(10 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s for a small heating surface of 10 mm×10 mm which is placed at the bottom surface of horizontal rectangular channel. The high pressure in the channel is observed at collapse of the coalescing bubbles and it is closely related the size of coalescing bubbles. Periodic pressure waves are observed in MEB and the heat flux increases linearly in proportion to the pressure frequency. The frequency is considered the frequency of liquid-solid exchange on the heating surface. For the large sized heating surface of 50 mm length×20 mm width, the maximum heat flux obtained is 500 W/cm² (5 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s. This is considerably higher heat flux than the conventional cooling limit in power electronics. It is difficult to remove the high heat flux by MEB for a longer heating surface than 50 mm by single channel type. A model of advanced cooling device is introduced for power electronics by subcooled flow boiling with impinging jets. Themaxumum cooling heat flux is 500 W/cm² (5 MW/m²). Microbubble emission boiling is useful for a high heat flux transport technology in future power electronics used in a fuel-cell power plant and a space facility.     

Measurement of spray boiling refrigerant coefficients in an integral-fin tube bundle segment simulating a full bundle

Outside (refrigerant) boiling coefficients for a combination of spray and drip boiling for a low pressure refrigerant have been obtained from overall heat transfer coefficients in a 1024 fins per meter tube bundle segment. The tubes were heated by water on the inside; liquid refrigerant was sprayed and/or dripped on the outside. Also, refrigerant vapor was supplied at the bottom of the bundle segment. This configuration simulates an actual flooded evaporator under spray boiling conditions. The dripping corresponds to liquid film falling from upper rows while the inlet vapor is equivalent to the vaporized refrigerant rising from lower tubes; the refrigerant vapor can influence heat transfer performance by the combined effects of gas convection and liquid shear on the tubes. For a nominal heat flux of 23,975 W/m², a bundle average outside heat transfer coefficient of 8522 W/m² °C, based on nominal tube outer diameter, was found at an average bundle vapor mass flux equal to 12.4 kg/s m². The distributor plate below the bundle enhanced the heat transfer, especially at lower vapor mass fluxes, by providing a level of liquid hold-up just below the bottom tube row.     

Bubble — bubbles — boiling

A short overview of boiling research in microgravity performed during the past two decades is subject of this presentation. The research was concentrated on pool boiling without applying any external forces. The objective of this research was to answer the questions: Is boiling an appropriate mechanism of heat transfer in space applications, and how do heat transfer and bubble dynamics behave without buoyancy, shear or electrical field forces? Is bubble dynamics itself being able to maintain heat transfer during boiling? The correlations used today to calculate heat transfer coefficients for practical applications in pool boiling are more or less based on the assumption that buoyancy detaches the bubbles from the heating surface and carry vapor with hot liquid away. With this model heat transfer would break down in microgravity. That’s why microgravity itself is an outstanding environment to study boiling in order to gain a better understanding of the complex interrelated physical mechanisms. Various carrier systems that allow simulation of microgravity could be used, such as drop tower ZARM, drop shaft JAMIC, parabolic trajectories with NASA’s aircraft KC-135, ballistic rockets TEXUS, and finally three Space Shuttle missions. As far as the possibilities of the respective mission allowed, a systematic research program [1] was followed, which was continuously adjusted and updated to new results and parameters. We discuss the hydrodynamic and thermal behavior of single bubbles, the dynamics during coalescence processes and the interaction of bubbles at the hot wall during boiling with the processes: boundary layer superheat, nucleation, bubble growth, detachment and departure. Surprising results have been obtained, that not only saturated and subcooled boiling can be maintained in microgravity, but also that at lower heat fluxes an enhancement of heat transfer compared to terrestrial was observed, while most today used empirical correlations show a strong decrease extrapolated to lower gravity values. However, it must be pointed out that also the maximum accessible heat flux, the so called “critical heat flux”, is higher than predicted by present used relations, but as far as reliable values are available, reduced by about 50 % compared to terrestrials. With the simultaneously observed bubble dynamics the heat transfer results can be interpreted and both give rise to a better physical understanding of the boiling process.     

New type of dry out crisis in free falling boiling liquid films

Here we present measurements of heat-transfer rate and critical heat flux as well as high-speed visualization at intensively evaporating and boiling falling liquid film flow. Research were carried-out at heated surfaces 67 mm wide and 20, 42, 64 mm long along the film flow over the range of inlet Reynolds number from 100 to 2000. Appreciably different crisis phenomena scenarios are observed depending on the heated surface length. Direct experimental measurements and visualization have shown the existence of previously unexplored surface dry out crisis development regime which is characterized by the upstream extrusion of the bubble boiling from the heated surface with the drying front. This type of crisis occurs under the certain range of the operating conditions and heated surface lengths. When the critical heat flux density occurs, large dry spots merge at the lower part of the stream. As a result of the dry spots merging, unstable temperature disturbance is formed. Subsequently it spreads over the whole surface, causing its drying, and critical heat flux is no longer determined by known calculation dependencies and is characterized by significantly smaller values. At such regimes critical heat flux is controlled not by hydrodynamic boiling crisis, but by equilibrium heat flux at which dry spots become unstable. This undesired critical heat flux reduction is potentially avoidable if measures can be taken for artificial liquid redistribution in transversal direction in order to decrease dry spots initial size (thus increase equilibrium heat flux).     

Review on pool boiling heat transfer of carbon dioxide

Although application of carbon dioxide as working fluid in many fields of refrigeration technology has been recommended very often in the recent past, the data on nucleate boiling heat transfer of carbon dioxide in free convection are very scarce in the open literature and new investigations are almost entirely focussed on forced convective flow boiling. In the interpretation of the respective results, heat transfer to carbon dioxide is often characterized as being superior to other refrigerants due to the outstandingly favourable thermophysical properties of carbon dioxide for boiling heat transfer. Different from this view, the discussion of recent results on pool boiling heat transfer of carbon dioxide in this review demonstrates that the high heat transfer coefficients measured for carbon dioxide in comparison to hydrocarbon or halocarbon refrigerants are mainly due to the fact that application of carbon dioxide is mostly envisaged for conditions where reduced saturation pressure p*=p_s/p_c (p_c, critical pressure) is higher than for common refrigerants.

In the first part of the review, the three main influences—by heat flux, saturation pressure and fluid properties—on pool boiling of carbon dioxide are discussed using recent measurements for CO₂ by Kotthoff et al. [S. Kotthoff, U. Chandra, D. Gorenflo, A. Luke, New measurements of pool boiling heat transfer for carbon dioxide in a wide temperature range, Proceedings of the Sixth IIR-Gustav Lorentzen Conference, Glasgow, 2004 [paper 2/A/3.30]; see also S. Kotthoff, U. Chandra, D. Gorenflo, Neue Messungen zum Behältersieden von Kohlendioxid in einem grösseren Temperaturbereich, DKV-Tagungsbericht 22 (2004) [Bd.II. 1] 233–256 and other organic substances (Gorenflo et al.) [D. Gorenflo, S. Kotthoff, U. Chandra, New measurements of pool boiling heat transfer with hydrocarbons and other organics for update of VDI—Heat Atlas calculation method, Proceedings of the Sixth IIR-Gustav Lorentzen Conference, Glasgow, 2004 [paper 1/C/1.00]; Kotthoff and Gorenflo, [S. Kotthoff, D. Gorenflo, Influence of the fluid on pool boiling heat transfer of refrigerants and other organic substances, Proceedings of the IIR-Commission B1 Conference, Vicenza, 2005 [paper #TP-98]. In the second part, a comparison is given with the few former data available and with new results of Loebl and Kraus [S. Loebl, W.E. Kraus, Pool boiling heat transfer of carbon dioxide on a horizontal tube, Proceedings of the Sixth IIR-Gustav Lorentzen Conference, Glasgow, 2004 [paper 1/A/1.20]; S. Loebl, W.E. Kraus, Zum Wärmeübergang bei der Verdampfung von Kohlendioxid am horizontalen Rohr, DKV-Tagungsbericht 22 (2004) [Bd.II.1] 219–232 on the influence of the heating wall (material and surface roughness). Finally, analogies between nucleate pool boiling and new flow boiling data are shown for those domains of flow boiling in which nucleation provides the dominant contribution to heat transfer and convective effects are of secondary importance.     

Experimental and numerical study of the compact heat exchanger with different microchannel shapes

Experimental and numerical analysis of heat transfer and fluid flow in the compact heat exchanger has been done in this paper. In an open circuit wind tunnel, developed on purpose for this investigation, the measurement of working media temperatures and mass flow rates for heat exchanger with microchannel coil has been accomplished. In accordance with the heat exchangers used for experiments, numerical 3D simulation of adequate geometry shapes has been done. With utilization of air/water side numerical simulation, more detailed results have been achieved in relation to the simulation that assumes constant temperature or constant heat flux on the pipe wall. Good agreement between experimentally measured and numerically calculated results has been accomplished. The influence of different microchannel shapes on heat transfer effectiveness and pressure drop has been studied numerically. Comparison of results has been made accompanied by the discussion and final conclusions.     

Experimental Study of the Relation Between Heat Transfer and Flow Behavior in a Single Microtube

The flow boiling heat transfer in microchannels have become important issue because it is extremely high-performance heat exchanger for electronic devices. For a detailed study on flow boiling heat transfer in a microtube, we have used a transparent heated microtube, which is coated with a thin gold film on its inner wall. The gold film is used as a resistance thermometer to directly evaluate the inner wall temperature averaged over the entire temperature measurement length. At the same time, the transparency of the film enables the observation of fluid behavior. Flow boiling experiments have been carried out using the microtube under the following conditions; mass velocity of 105 kg/m² s, tube diameter of 1 mm, heat flux in the range of 10 ~ 380 kW/m² s, and the test fluid used is ionized water. Under low heat flux conditions, the fluctuations in the inner wall temperature and mass velocity are closely related; the frequency of these fluctuations is the same. However, the fluctuations in the inner wall temperature and heat transfer coefficient are found to be independent of the fluctuation in the mass velocity under high heat flux conditions.     

Influence of the heat flux in mixture boiling: experiments and correlations

Heat transfer at nucleate pool boiling of the binary and ternary refrigerant mixtures R404A, R407C and R507 at the outside of a horizontal tube with emery ground surface has been investigated in a wide range of pressures and heat fluxes. Together with experimental data of Bednar and Bier for wide boiling ethane/n-butane mixtures, the results of these comparatively narrow boiling mixtures are used to investigate the influence of heat flux q on the heat transfer coefficient as predicted by various correlations for nucleate boiling of mixtures. At comparatively high saturation pressures with experimental -values markedly smaller than the molar average of the pure components, the ,q-relationships predicted differ significantly from the experimental, particularly for wide boiling mixtures.