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.     

A review of hydrocarbon two-phase heat transfer in compact heat exchangers and enhanced geometries

Hydrocarbons are considered as alternative fluids for refrigeration, air-conditioning and heat pump applications. Pure butane, propane or their mixtures can be adopted, but due to their flammable properties, the systems have to be designed in such a way that the refrigerant charge is minimized. Therefore, compact heat exchangers and enhanced geometries are adopted in such systems. In this paper, the current state of the art for two-phase heat transfer calculations for pure hydrocarbons and their mixtures is reviewed and analysed. Recommendations are proposed for estimating evaporation and condensation heat transfer in various geometries including enhanced tubes as well as compact heat exchangers.     

State of the art in pool boiling heat transfer of new refrigerantsEtat du calcul pour le transfert de chaleur en ébullition libre de nouveaux frigorigènes

The predictive methods for the calculation of the heat transfer coefficient α with pool boiling are important tools for the optimum design of the evaporator and for the successful operation of refrigeration units. The method given in the VDI Heat Atlas is discussed as an example of the currently available methods/ and results of recent experimental investigations on nucleate boiling of partly fluorinated hydrocarbons (HFCs) and of hydrocarbons (HCs) are added covering those parts where the predictive methods should be improved, namely boiling of mixtures, influence of surface structure and material of the heating wall, and influence of additional flow of bubbles and liquid in tube bundles.     

Pool boiling of ammonia/water and its pure components: Comparison of experimental data in the literature with the predictions of standard correlations

Although ammonia/water has been used for decades as a working pair in absorption cycles for industrial refrigeration, very limited data are available on boiling heat transfer of this mixture. The intention of this work is to carry out a bibliographic revision of the information available in the open literature about nucleate pool boiling of the ammonia/water mixture and its pure components. The experimental data have been compared with existing prediction correlations for the pure components and also for their mixtures.For water, all the pure component pool boiling correlations gave similar predictions and were in good agreement with experimental data. For ammonia the prediction of the correlation and the experimental data showed more differences.At a given heat flux, the experimental data show that the mixture pool boiling heat transfer coefficient is lower than that obtained with pure components. Three of the well-known correlations for mixtures were compared against the experimental data. None of these correlations provided a good prediction of the mixture pool boiling heat transfer coefficient over a wide range of mass fraction. Furthermore, a new correlation has been proposed.     

Modelling heat and mass transfer in frozen foods: a review

This paper reviews mathematical methods for modelling heat and mass transfer during the freezing, thawing and frozen storage of foods. It starts by considering the problems in modelling heat transfer controlled freezing (the Stefan problem): release of latent heat, sudden changes in thermal conductivity. The author gives a unified overview of the common numerical methods: finite difference, finite element and finite volume. Mass transfer is then considered, involving different phenomena and approaches for dense and porous foods. Supercooling, nucleation and trans-membrane diffusion effects during freezing, and recrystallization during frozen storage are considered next. High pressure thawing and thawing are considered in view of their recent popularity. Finally, the paper offers a brief look at mechanical stresses during freezing, a much neglected area. It is concluded that while modelling heat transferred controlled freezing is a settled problem, much work remains to be done in modelling associated phenomena in order to gain the ability to predict changes in food quality at the micro-level.     

Assessment of boiling heat transfer correlations in the modelling of fin and tube heat exchangers

A new way to assess the performance of refrigeration system models is presented in this paper, based on the estimation of cycle parameters, such as the evaporation temperature which will determine the validity of the method. This paper is the first of a series which will also study the influence of the heat transfer coefficient models on the estimation of the refrigeration cycle parameters. It focuses on fin and tube evaporators and includes the dehumidification process of humid air. The flow through the heat exchanger is considered to be steady and the refrigerant flow inside the tubes is considered one-dimensional. The evaporator model is discretised in cells where 1D mass, momentum and energy conservation equations are solved by using an iterative procedure called SEWTLE. This procedure is based on decoupling the calculation of the fluid flows from each other assuming that the tube temperature field is known at each fluid iteration. Special attention is paid to the correlations utilised for the evaluation of heat transfer coefficients as well as the friction factor on the air and on the refrigerant side. A comparison between calculated values and measured results is made on the basis of the evaporation temperature. The experimental results used in this work correspond to an air-to-water heat pump and have been obtained by using R-22 and R-290 as refrigerants.     

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.     

Assessment of boiling and condensation heat transfer correlations in the modelling of plate heat exchangers

This paper studies refrigeration cycles in which plate heat exchangers are used as either evaporators or condensers. The performance of the cycle is studied by means of a method introduced in previous papers which consists of assessing the goodness of a calculation method by looking at representative variables such as the evaporation or the condensation temperature depending on the case evaluated. This procedure is also used to compare several heat transfer coefficients in the refrigerant side. As in previous works the models of all the cycle components are considered together with the heat exchanger models in such a way that the system of equations they provide is solved by means of a Newton–Raphson algorithm. Calculated and measured values of the evaporation and the condensation temperatures are also compared. The experimental results correspond to the same air-to-water heat pump studied in other papers and they have been obtained by using refrigerants R-22 and R-290.     

NH3 in-tube condensation heat transfer and pressure drop in a smooth tube

This paper presents an overview of the issues and new results for in-tube condensation of ammonia in horizontal round tubes. A new empirical correlation is presented based on measured NH₃ in-tube condensation heat transfer and pressure drop by Komandiwirya et al. [Komandiwirya, H.B., Hrnjak, P.S., Newell, T.A., 2005. An experimental investigation of pressure drop and heat transfer in an in-tube condensation system of ammonia with and without miscible oil in smooth and enhanced tubes. ACRC CR-54, University of Illinois at Urbana-Champaign] in an 8.1 mm aluminum tube at a saturation temperature of 35 °C, and for a mass flux range of 20–270 kg m⁻² s⁻¹. Most correlations overpredict these measured NH₃ heat transfer coefficients, up to 300%. The reasons are attributed to difference in thermophysical properties of ammonia compared to other refrigerants used in generation and validation of the correlations. Based on the conventional correlations, thermophysical properties of ammonia, and measured heat transfer coefficients, a new correlation was developed which can predict most of the measured values within ±20%. Measured NH₃ pressure drop is shown and discussed. Two separated flow models are shown to predict the pressure drop relatively well at pressure drop higher than 1 kPa m⁻¹, while a homogeneous model yields acceptable values at pressure drop less than 1 kPa m⁻¹. The pressure drop mechanism and prediction accuracy are explained though the use of flow patterns.     

Current and future prospects of enhanced heat transfer in ammonia systems

In the last decade a moderate headway has been made in the application of enhanced surface heat exchangers in ammonia refrigeration systems. This has been a result of the persistent issue of ozone and global warming which has resulted in keen interest in natural refrigerants such as ammonia that has played a prominent role in the refrigeration industry for years, particularly in the field of food, beverage and marine. The only drawback with ammonia is the toxicity; hence, if smaller heat exchangers could be introduced in order to reduce ammonia charge, this negative aspect about ammonia can be addressed to a great extent. In order to achieve this goal, novel and compact heat exchangers with enhanced surfaces have to be introduced. This paper presents an over view of the status of ammonia as a refrigerant and discusses the present and the future trends in the development of compact heat exchangers for use in ammonia refrigeration.     

A review on surface heat and mass transfer coefficients during air chilling and storage of food products

Heat and mass transfer coefficients are needed for the mathematical simulation of air chilling and storage of solid food products. Average transfer coefficient values and distributions around cylinders are now quite well known while it is not the case for solids of more complex shapes. CFD models are more and more often used to calculate transfer coefficient values on a single food product or packaging. Experimental knowledge is reviewed and the way it can be used by scientists and engineers to determine their own transfer coefficient values is illustrated. The potentials and limitations of present CFD models are illustrated and future improvements are also discussed.     

Flow boiling heat transfer characteristics of CO2 at low temperatures

This paper presents an overview of the flow boiling heat transfer characteristics and the special thermo-physical properties of CO₂ at low temperatures (down to −30 °C). Subsequently, the boiling heat transfer of CO₂ at low temperatures is experimentally investigated in a horizontal tube with inner diameter of 4.57 mm. Due to the large surface tension, the boiling heat transfer coefficient of CO₂ is found to be much lower at low temperatures but it increases with vapour quality (until dryout), which is contrary to the trend at high temperatures around 0 °C. None of the empirical correlations from open literature were able to predict the boiling heat transfer coefficient for CO₂ in good agreement with the experimental data, suggesting the need for further research in this area.     

Pool boiling heat transfer of carbon dioxide on a horizontal tube

Carbon dioxide is again becoming an important refrigerant. While the thermophysical properties are well known there is a lack of data on its heat transfer characteristics.

In this study, heat transfer coefficients for nucleate boiling of carbon dioxide are determined using a standard apparatus for the investigation of pool boiling based on a set-up from Karlsruhe [D. Gorenflo, J. Goetz, K. Bier. Vorschlag für eine Standard-Apparatur zur Messung des Wärmeübergangs beim Blasensieden. Wärme-und Stoffübertragung 16 (1982), 69–78; J. Goetz, Entwicklung und Erprobung einer Normapparatur zur Messung des Wärmeübergangs beim Blasensieden. Dissertation Universität Karlsruhe (1980).] and built at our institute. Electrically heated horizontal cylinders with an outer diameter of 16 mm and a length of 100 mm are used as heating elements. Measurements with constant heat flux are performed for different wall materials and surface roughnesses. The heat transfer is investigated within the pressure range of 0.53≤ p ≤1.43 MPa (0.072≤ p/p_c ≤0.190) and a temperature range of −56≤ t ≤−30 °C, respectively. Heat fluxes of up to 80,000 W m⁻² are applied.

The influences of wall material and roughness on the heat transfer coefficient are evaluated separately. The obtained coefficients are compared to generally accepted correlations and to experimental results of other authors, who used similar configurations with copper tubes and carbon dioxide. These are the only previous experimental data, which could be found. Results for copper, stainless steel and aluminium as wall materials are presented.     

Experimental correlation of pool boiling heat transfer for HFC134a on enhanced tubes: Turbo-E

The objectives of this paper are to study the heat transfer characteristics for enhanced surface tubes in the pool boiling and to provide a guideline for the design conditions for the evaporator using HFC134a. The shape of tube surfaces, the wall superheat, and the saturation temperature are considered as the key parameters. Copper tubes (d_o = 19.05 mm) are treated with different helix angles and the saturation temperatures are controlled from 3 to 16 °C. It is found that the pool boiling heat transfer coefficient decreases with increasing the wall superheat. It is also found that boiling heat transfer coefficients for Turbo-II and Turbo-III are 1.5–3.0 times and 1.2–2.0 times higher than that for Turbo-I without the helix angle, respectively. The higher heat transfer performance from Turbo-II and Turbo-III can be explained by the “bubble detention” phenomenon on the surface without the helix angle for the Turbo-I. The experimental correlations for the pool boiling heat transfer on the present enhanced tubes without (Type I) and with the helix angle (Type II and Type III) are developed with the error bands of ±30%, respectively.     

Condensation of hydrocarbons – A review

Hydrocarbons are one of the candidates for refrigerants of next generation heat pump and refrigeration systems. Although the hydrocarbons have superior thermophysical properties as a refrigerant to fluorocarbons and are widely used in domestic refrigerators, their flammability prevents their wide application to larger systems, such as residential and packaged air conditioners, car air conditioners, heat pumps, etc. In this paper, recent studies on condensation of hydrocarbons are reviewed since it is one of the key technologies. For in-tube condensation, heat transfer coefficients of smooth tubes are correlated well with previously proposed equation developed with data of the fluorocarbons. On the other hand, few data are available for enhanced tubes. In the case of condensation on a horizontal tube, heat transfer of smooth tube is explained well by the Nusselt theory. However, different heat transfer characteristics appear for enhanced tubes. Since blended hydrocarbons would be used to obtain suitable properties, models of mixed vapor condensation are also reviewed.     

Nucleate boiling heat transfer coefficients of pure halogenated refrigerants

Nuclate pool boiling heat transfer coefficients (HTCs) of HCFC123, CFC11, HCFC142b, HFC134a, CFC12, HCFC22, HFC125 and HFC32 on a horizontal smooth tube of 19.0 mm outside diameter have been measured. The experimental apparatus was specially designed to accomodate high vapor pressure refrigerants such as HFC32 and HFC125 with a sight glass. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool of 7 °C. Test results showed that HTCs of HFC125 and HFC32 were 50–70% higher than those of HCFC22 while HTCs of HCFC123 and HFC134a were similar to those of CFC11 and CFC12 respectively. It was also found that nucleate boiling heat transfer correlations available in the literature were not good for certain alternative refrigerants such as HFC32 and HCFC142b. Hence, a new correlation was developed by a regression analysis taking into account the variation of the exponent to the heat flux term as a function of reduced pressure and some other properties. The new correlation showed a good agreement with all measured data including those of new refrigerants of significantly varying vapor pressures with a mean deviation of less than 7%.     

Forced convective boiling heat transfer of R-410A in horizontal minichannels

Convective boiling heat transfer experiments were performed in horizontal minichannels with binary mixture refrigerant, R-410A. The test section is made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm and with lengths of 1500 mm and 3000 mm, respectively, and is uniformly heated by applying electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 10–30 kW m⁻², a mass flux range of 300–600 kg m⁻² s⁻¹, and quality ranges of up to 1.0. The experimental results were mapped on Wang et al.'s (C.C. Wang, C.S. Chiang, D.C. Lu, Visual observation of two-phase flow pattern of R-22, R-134a, and R-407C in a 6.5-mm smooth tube, Experimental, Thermal and Fluid Science 15 (1997) 395–405) and Wojtan et al.'s (L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes: part I – a new diabatic two-phase flow pattern map, International Journal of Heat and Mass Transfer 48 (2005) 2955–2969) flow pattern maps to observe the flow regimes. Laminar flow appears in flow with minichannels. A new boiling heat transfer coefficient correlation based on the superposition model for R-410A was developed with 11.20% mean deviation; it showed a good agreement between the measured data and the calculated heat transfer coefficients.     

Prediction methods for pool boiling heat transfer: A state-of-the-art review

Prediction of heat transfer in the evaporator of a refrigeration unit should be as accurate as possible for the successful operation of the whole appliance. The predictive methods used at present are empirical or semi-empirical, particularly for the heat transfer conditions relevant in practice, because theoretically consistent calculation of the heat transfer coefficient in nucleate boiling is not yet possible. One of these which is included in the VDI Heat Atlas and has been updated recently, is presented in the first main part below and is compared with experimental data for 55 fluids. In the second part, eight more prediction methods from the literature are tested using the same experimental database, and the deviations between measurement and calculation are discussed in detail together with the different results calculated with the various methods.     

A semi-theoretical model for predicting refrigerant/lubricant mixture pool boiling heat transfer

This paper outlines the framework of a semi-theoretical model for predicting the pool boiling heat transfer of refrigerant/lubricant mixtures on a roughened, horizontal, flat pool-boiling surface. The predictive model is based on the mechanisms involved in the formation of the lubricant excess layer that exists on the heat transfer surface. The lubricant accumulates on the surface in excess of the bulk concentration via preferential evaporation of the refrigerant from the bulk refrigerant/lubricant mixture. As a result, excess lubricant resides in a thin layer on the surface and influences the boiling performance, giving either an enhancement or degradation in heat transfer. A dimensionless excess layer parameter and a thermal boundary layer constant were derived and fitted to data in an attempt to generalize the model to other refrigerant/lubricant mixtures. The model inputs include transport and thermodynamic refrigerant properties and the lubricant composition, viscosity, and critical solution temperature with the refrigerant. The model predicts the boiling heat transfer coefficient of three different mixtures of R123 and lubricant to within ±10%. Comparisons of heat transfer predictions to measurements for 13 different refrigerant/lubricant mixtures were made, including two different refrigerants and three different lubricants.