Contribution to the static heat transfer to boiling liquid helium

Aluminium is now being used as a stabilizing material for superconducting materials. With regard to its use as a stabilizer, the heat transfer properties to boiling helium of 99.9999% pure aluminium are measured and compared with those of 99.999% pure aluminium and OFHC copper     

Nucleate boiling heat transfer coefficients of flammable refrigerants

Nucleate boiling heat transfer coefficients (HTCs) of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) on a horizontal smooth tube of 19.0 mm outside diameter have been measured. The experimental apparatus was specially designed to accommodate high vapor pressure refrigerants such as propylene and propane 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 kW m⁻² to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool temperature of 7 °C. Test results exhibited a typical trend that HTCs of flammable refrigerants increase with increasing vapor pressure. Existing nucleate boiling heat transfer correlations showed up to 80% deviation as compared to the present data. Hence a new correlation was developed through a regression analysis taking into account dimensionless variables affecting nucleate boiling heat transfer. The new correlation showed a good agreement with data for flammable refrigerants as well as halogenated refrigerants with a deviation of 5.3%.     

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 convection heat transfer to liquid helium I in the nucleate boiling region

Heat transfer and critical heat fluxes to helium boiling in a 2 mm id copper tube (100 mm long) were measured in the pressure range 1.1–1.5 atm and at mass velocities 18–96 kg m^?2s^?1. Corresponding Reynolds numbers are (1.2–6.2) × 10⁴. Experimentally obtained heat transfer coefficients show satisfactory agreement with those calculated according to the Kutateladze equation but with less pronounced pressure dependence. It was found that in the boiling region developed quality did not influence the heat transfer coefficient. An expression was obtained, which describes with ±10% error, the dependence of critical heat flux on mass flow rate in the pressure range 1.1–1.5 atm and mass quality 0.33–0.6.     

Nucleate boiling of a liquid

A balance method is described for the analysis of nucleate pool boiling of a liquid.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 23, No. 6, pp. 1071–1077, December, 1972.     

Forced convection heat transfer to boiling helium in a tube

This paper describes a study carried out to obtain the heat transfer characteristics of liquid helium l in a vertical tube under forced convection conditions. The test section was a straight stainless-steel tube with a length of 8.5 cm and an id of 0.109 cm. Helium pressures ranged from 1.1 to 1.9 atm (0.11 to 0.19 MN m^?2) and qualities (vapour mass fractions) ranged from zero to 1.Results were correlated using conventional equations and compared with the data already obtained. The hysteresis observed in the nucleate boiling region is qualitatively discussed. The quality dependence of two-phase flow heat transfer was clearly observed during an increase, but not during a decrease, of heat flux or quality.     

Recovery heat flux from silent and noisy film boiling states in saturated liquid He II

Heat transfer into saturated He II was investigated in an experiment which simulates the use of He II as coolant for the stabilization of superconducting magnets. One must distinguish two critical values of the recovery heat flux density (ie recovery from film boiling to non-boiling), namely for recovery from silent film boiling and recovery from noisy film boiling. There are also two threshold values of the power of short-time heat pulses which trigger a transition from the non-boiling heat transfer into film-boiling. The increased cooling capacity of He II at about 1.9 K seems to compensate for the reduction of the thermal parameters of the stabilizing material at the low temperature.     

Measurement of the transient heat transfer to liquid helium from a thin metal film

A method to measure transient heat transfer to liquid helium from a thin metal film heater under the condition of pulsed heating during τ ≤ 400 ns is proposed. The experimental equipment used for the measurements is described. The method is based on the comparison of heat pulses transfered from the heater into a monocrystal substrate which is surrounded either by vacuum or by liquid helium. The method can also be used to investigate the heat flux density transmitted into liquid helium over a wide region of thermal loads. Experimental results showing the heat flux density radiated from a Cu heater into liquid helium at 3.8 K as a function of the electric power fed into the heater by pulses of 200–400 ns duration are demonstrated.     

Phase slip and heat transfer to the liquid in film boiling of a cryogenic liquid in piston flow

Published data are examined and results are given from a survey of experiments on phase slip and heat transfer to liquids.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 3, pp. 442–448, September, 1980.     

Characteristic curve of helium pool boiling

The results of an experimental investigation on helium pool boiling over a wide range of temperature differences covering nucleate, transition, and film regions are presented. Heating surfaces under investigation were made of different materials and were processed with various degrees of roughness. Helium boiling curve on stainless steel has an anomalous character: heat flux in the transition region increases when temperature difference rises and there is no nucleate boiling zone on the surface with the lowest degree of roughness. A comparative analysis of the results presented with available data is also carried out.     

Nucleate boiling heat transfer coefficients of flammable refrigerants on various enhanced tubes

In this study, nucleate boiling heat transfer coefficients (HTCs) of five flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) were measured at the liquid temperature of 7 °C on a low fin tube of 1023 fins per meter, Turbo-B, and Thermoexcel-E tubes. All data were taken from 80 to 10 kW m⁻² with an interval of 10 kW m⁻² in the decreasing order of heat flux. Flammable refrigerants' data showed a typical trend that nucleate boiling HTCs obtained on enhanced tubes also increase with the vapor pressure. Fluids with lower reduced pressure such as DME, isobutene, and butane took more advantage of the heat transfer enhancement mechanism of enhanced tubes than those with higher reduced pressure such as propylene and propane. Finally, Thermoexcel-E showed the highest heat transfer enhancement ratios of 2.3–9.4 among the tubes tested due to its sub-channels and re-entrant cavities.     

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.     

Correlation of heat transfer data on stable film pool boiling at vertical surfaces for free convection

Experimental heat transfer data on film boiling of a liquid at the vertical surfaces of a heater are analyzed and correlated.     

Analysis of helium II film boiling

The heat flux removed in the boundary transition from a vapour film to helium II is studied. It was found that the properties of the heat flux can be essentially explained physically by non-equilibrium molecular-kinetic effects at the vaporization condensation interface. A method of calculating the value of the heat flux is proposed.     

HFE-7100 pool boiling heat transfer and critical heat flux in inclined narrow spaces

Experiments were performed to examine the pool boiling heat transfer and critical heat flux on a smooth copper circular surface, confined by a face-to-face parallel unheated surface, by changing both the orientation and the gap between the surfaces. Pool boiling data at atmospheric pressure were obtained for saturated HFE-7100. The gaps between the boiling surface and adiabatic one were 0.5, 1.0, 2.0, 3.5, 5.0, 10.0 and 20.0 mm. For each configuration, boiling curves were obtained up to the thermal crisis. The surface orientations investigated were 0° (horizontal upward surface), 45°, 90° and 135°. It was observed that the heat transfer coefficient improves at low wall superheat when the distance between heated and unheated surfaces decreases. However, at high wall superheat, a drastic reduction in heat transfer as well as CHF appears for confined boiling. For a fixed channel width, the CHF value strongly depends on the channel orientation, assuming a maximum value for near-vertical channels with up-facing heated surface. CHF data were compared with various literature correlations; a new empirical correlation that takes into account channel gap effects is proposed for horizontal confined surfaces.     

Influence of thermophysical properties on pool boiling heat transfer of refrigerants

The correct prediction of the heat transfer performance of the boiling liquid within the evaporator of a refrigeration unit is one of the essential features for the successful operation of the whole unit. A theoretically consistent calculation method for the heat transfer coefficient α in nucleate boiling, which should be based on the physical phenomena connected with vapour bubbles growing, departing and sliding on the wall and with the interactions of bubbles and of neighbouring nucleation sites within the microstructure of the heating surface, does not yet exist, despite the increasing number of papers on the subject in the recent past. Instead, the predictive methods for α available at present are empirical or semiempirical, especially for heat transfer conditions relevant in practice. Many of these correlations have been established in the form of power laws in which the relative influences of the main groups of variables on α are treated by separate factors. One of these may stand for the influence of the thermophysical properties of the boiling liquid or these properties will be included in several of the factors.New experimental results are presented for pool boiling heat transfer from a single horizontal copper tube (8 mm OD) to HFC-refrigerants (R32, 125, 134a, 143a, 152a, 227ea) and hydrocarbons (propane, i-butane). The results are compared to experimental data from the literature, and methods are discussed, how to incorporate the data in semiempirical correlations to describe the influence of the thermophysical properties of the fluids on the heat transfer performance.