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.     

Boiling hysteresis at low temperature on enhanced tubes

The boiling hysteresis phenomenon is studied for a real scale enhanced evaporator tube (2 m long Turbo-B type) with R134a refrigerant used in the flooded evaporator of a centrifugal brine chiller for the ice-making facility. Unlike previous studies of the boiling heat transfer with uniform heat flux and uniform wall temperature, the wall temperature varies along the tube in the present experiment. To see if the similar hysteresis occurs as in the case of uniform wall temperature, a careful control of refrigerant temperature and heat flux is made. We have found hysteresis of the temperature overshoot (TOS) at the onset of nucleate boiling initially at the inlet section of the tube, before it gradually moved downstream section of the tube until the nucleate boiling occupied the whole section of the tube as the inlet temperature increased. The hysteresis became stronger at low refrigerant temperatures. The decreasing trend of heat flux after the contents of the whole tube boiled was different from the increasing trend. This paper provides a guideline how to design the evaporator in order to avoid the abnormal operation of the chillers.     

Flow boiling heat transfer of carbon dioxide with PAG-type lubricating oil in pre-dryout region inside horizontal tube

A flow boiling heat transfer model for horizontal tubes is proposed for CO₂ with entrained polyalkylene glycol (PAG) type lubricating oil in the pre-dryout region. A general power law-type model with a power number of 3 is used together with the average thermodynamic properties of the CO₂–oil mixture. A convective enhancement factor (F) is recommended according to the relationship between the Lockhart–Martinelli parameter and the ratio α_tp/α_l, which was obtained based on previous experimental results for CO₂ and oil. A new suppression factor (S) is introduced that comprises a suppression term for forced convection and oil concentration term for bubble generation. A comparison of six correlations showed that the proposed correlation can depict the influence of the mass and heat fluxes on both nucleate and convection boiling reasonably well.     

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.     

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.     

Critical heat flux of binary mixtures in pool boiling and its correlation in terms of Marangoni number

Critical heat flux (CHF) in nucleate pool boiling of binary mixtures was newly measured with a horizontal platinum wire, 0.5 mm in diameter, and heated by DC, over the full range of concentrations. Seven mixtures were selected with the intent to cover various types of mixtures: methanol/water, ethanol/water, methanol/ethanol, ethanol/n-butanol, methanol/benzene, benzene/n-heptane and water/ethylene glycol, each in the saturated state at atmospheric pressure. Total 311 raw CHF data were obtained at 75 concentrations including pure components.Aqueous mixtures of methanol and ethanol revealed significant increase of CHF compared to either CHF linearly interpolated between pure components or CHF predicted from a single component correlation with use of the mixture properties. Three organic mixtures showed more or less the same level as an interpolated CHF, while the remaining two mixtures of methanol/benzene and water/ethylene glycol gave the reduced CHF by 20% and 50% at most, respectively. Marangoni number was introduced as a controlling variable to explain the observed increased, invariable, or reduced CHF, and an empirical correlation was developed.     

Numerical analysis of regenerators of free-piston type Stirling engines using Lagrangian formulation

The circular duct between the cylinder and displacer serves as a regenerator in free-piston Stirling engines. The cylinder wall is fixed and the displacer wall is in reciprocating motion during the steady operation of the engine. The basic equations of the working fluid and regenerative duct are derived using the Lagrangian method in terms of the displacement of the displacer, so that time does not appear in the equations. A relation is derived between the cylinder and displacer wall temperatures to obtain the initial wall temperature distributions. A computer program is written in and the governing equations, which include the pressure fluctuations due to the flow reversals, are solved numerically using a finite difference method. The results and discussion are presented.     

Effect of bulk lubricant concentration on the excess surface density during R134a pool boiling

This paper investigates the effect that the bulk lubricant concentration has on the non-adiabatic lubricant excess surface density on a roughened, horizontal flat pool-boiling surface. Both pool boiling heat transfer data and lubricant excess surface density data are given for pure R134a and three different mixtures of R134a and a polyolester lubricant (POE). A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of an R134a/POE lubricant mixture on a test surface. The lubricant is preferentially drawn out of the bulk refrigerant/lubricant mixture by the boiling process and accumulates on the surface in excess of the bulk concentration. The excess lubricant resides in an approximately 40 μm layer on the surface and influences the boiling performance. The lubricant excess surface density measurements were used to modify an existing dimensionless excess surface density parameter so that it is valid for different reduced pressures. The dimensionless parameter is a key component for a refrigerant/lubricant pool-boiling model given in the literature. In support of improving the boiling model, both the excess measurements and heat transfer data are provided for pure R134a and three R134a/lubricant mixtures at 277.6 K. The heat transfer data show that the lubricant excess layer causes an average enhancement of the heat flux of approximately 24% for the 0.5% lubricant mass fraction mixture relative to pure R134a heat fluxes between 5 and 20 kW/m². Conversely, both 1 and 2% lubricant mass fraction mixtures experienced an average degradation of approximately 60% in the heat flux relative to pure R134a heat fluxes between approximately 4 and 20 kW/m². This study is an effort toward generating data to support a boiling model to predict whether lubricants degrade or improve boiling performance.     

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.     

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.