Showing papers on "Subcooling published in 2001"

A Theoretical model to predict pool boiling CHF incorporating effects of contact angle and orientation

Abstract: A theoretical model is developed to describe the hydrodynamic behavior of the vapor-liquid interface of a bubble at the heater surface leading to the initiation of critical heat flux (CHF) condition. The momentum flux resulting from evaporation at the bubble base is identified to be an important parameter. A model based on theoretical considerations is developed for upward-facing surfaces with orientations of 0 deg (horizontal) to 90 deg (vertical). It includes the surface-liquid interaction effects through the dynamic receding contact angle. The CHF in pool boiling for water, refrigerants and cryogenic liquids is correctly predicted by the model, and the parametric trends of CHF with dynamic receding contact angle and subcooling are also well represented

Boiling Heat Transfer and Bubble Dynamics in Microgravity

Abstract: This article presents results for pool boiling heat transfer under microgravity conditions that the author and his team have gained in a succession of experiments during the past two decades. The objective of the research work was to provide answers to the following questions: Is boiling an appropriate mechanism of heat transfer for space application? How do heat transfer and bubble dynamics behave without the influence of buoyancy, or more general, without the influence of external forces? Is microgravity a useful environment for investigating the complex mechanisms of boiling with the aim of gaining a better physical understanding? Various carrier systems that allow simulation of a microgravity environment could be used, such as drop towers, parabolic trajectories with NASA’s aircraft KC-135, ballistic rockets such as TEXUS, and, more recently, three Space Shuttle missions. As far as the possibilities of the respective missions allowed, a systematic research program was followed that was continuously adjusted to actual new parameters. After a general survey concerning the importance of boiling for technical applications, an introduction is given especially for those individuals not closely familiar with the fields of microgravity and boiling. Surprising results have been obtained: not only that saturated pool boiling can be maintained in a microgravity environment, but also that at small heater surfaces and lower values of heat fluxes, even higher heat transfer coefficients have been attained than under terrestrial conditions. The bubble departure can be attributed to surface tension effects, to “bubble ripening” and coalescence processes. Under subcooled conditions only, thermocapillary flow was observed that transports the heat from the bubble interface into the bulk liquid, but does not enhance the heat transfer compared with boiling at saturated conditions. Direct electrical heated plane surfaces lead to a slow extension of dry spots to dry areas below bubbles, the increasing surface temperature suggesting transition to film boiling. The critical heat flux in microgravity is lower than under earth conditions, but considerably higher than the hitherto accepted correlations predict when extrapolated to microgravity. The nearly identical heat transfer coefficients received for nucleate boiling under microgravity as well as terrestrial conditions, and for both saturated and subcooled fluid states, also suggest identical heat transfer mechanisms. These results lead to the conclusion that the primary heat transfer mechanism must be strongly related to the development of the microlayer during bubble growth. Secondary mechanisms are responsible for the transport of enthalpy in form of latent energy of the bubbles and hot liquid carried with them. Under terrestrial conditions, that mechanism is caused by external forces such as buoyancy; under microgravity conditions, the self dynamics of the bubbles and/or thermocapillary flow under subcooled conditions are responsible. The results demonstrate clearly that boiling can be applied as a heat transfer mechanism in a microgravity environment and that microgravity is a useful means to study the physics of boiling.

Critical heat flux in subcooled flow boiling- an assessment of current understanding and future directions for research

Abstract: Multiphase Science and Technology article. The full article is published by Begell House Inc. and available at: http://www.begellhouse.com/ .

High-Speed photographic observation of flow boiling of water in parallel mini-channels

Abstract: The use of smaller passage dimensions is becoming more prevalent in flow boiling applications. Passages with hydraulic diameters on the order of 1mm provide higher heat transfer coefficients resulting in more compact heat exchanger arrangements. Passages with 1mm hydraulic diameters do not pose severe clogging or blocking constraints as opposed to micro-channels and have a less pronounced pressure drop penalty. The present paper explores the flow regimes during flow boiling of water in mini-channels. High-speed photography is used to obtain visual pictures of the flow phenomena from subcooled flow boiling all the way to critical heat flux conditions. The tests include one single-channel and a set of six parallel channels with electric heating from three sides of the channel. The top cover is made of Lexan to permit visual observations. The results are used to identify specific features of flow boiling in smaller diameter mini-channels. NOMENCLATURE Ac: Cross-Section area G: Mass flux of water through test section ( ) c A m m : Mass flow rate of water through test section (kg/s) Q”: Heat flux through channel walls to water (kW/m) TA: bulk temperature at the test section inlet (°C) TB: bulk temperature at the test section outlet (°C) TS: Average surface temperature of the test section (°C) x: vapor mass fraction at outlet ( ) fluid vapor m m INTRODUCTION The need to increase heat transfer coefficients on the evaporating liquid side in an evaporator is becoming increasingly important in many applications including automotive air conditioning, heat pipes, direct refrigeration cooling of electronic devices, and fuel cells. Although evaporation in small diameter channels received considerable attention in the 1960’s (for example, Bergles, 1964), its use in a compact evaporator configuration with multiple channels is currently receiving wide attention. The complexities associated with evaporation in multiple channel passages are not clearly understood. The present study focuses on providing an insight into the two-phase flow characteristics during evaporation of water in 1-mm hydraulic diameter, multiple channel, electrically heated evaporator section. LITERATURE REVIEW There are very few publications available in literature addressing the flow patterns in multi-channel evaporators with small diameter channels. Conventional compact evaporators are plate-fin type, with the evaporating liquid flowing between two parallel plates that have uniformly spaced bumps for brazing the plates together. These obstructions provide a heat transfer enhancement on the evaporation side. However, the evaporating liquid is free to flow across the plate width. The use of small diameter channels, each of 1-mm or less hydraulic diameter presents quite a different scenario. Here the pressure drop between the two manifolds is quite high and the evaporating liquid cannot flow across into another flow channel as in the case of a plate-fin type evaporator.

Flow Boiling Heat Transfer From Plain and Microporous Coated Surfaces in Subcooled FC-72

Abstract: The present research is an experimental study of subcooled flow boiling behavior using flat, microporous-enhanced square heater surfaces in pure FC-72. Two 1-cm 2 copper surfaces, one highly polished (plain) and one microporous coated, were flush-mounted into a 12.7 mm square, horizontal flow channel. Testing was performed for fluid velocities ranging from 0.5 to 4 m/s (Reynolds numbers from 18,700 to 174,500) and pure subcooling levels from 4 to 20 K. Both surfaces' nucleate flow boiling curves collapsed to one line showing insensitivity to fluid velocity and subcooling. The log-log slope of the microporous surface nucleate boiling curves was lower than the plain surface due to the conductive thermal resistance of the microporous coating layer. Both, increased fluid velocity and subcooling, increase the CHF values for both surfaces, however, the already enhanced boiling characteristics of the microporous coating appear dominant and require higher fluid velocities to provide additional enhancement of CHF to the microporous surface

Enhancing Small-Channel Convective Boiling Performance Using a Microporous Surface Coating

Abstract: Performance characteristics are experimentally determined for enhanced convective boiling of FC-87 in a horizontal, small-cross-sectional-area, single-channel heater The channel tested has a square cross section with a side length of 2 mm and a heated length of 8 cm. The heated surface of the channel is tested both with, and without, a microporous surface coating. Although few studies have been performed to quantify the effects of internal porous coatings on flow boiling, their application in this area is promising. The coating is shown to provide increases in both heat transfer coefficient and critical heat flux. Convective boiling curves are generated for mass fluxes from 500 to 5000 kg/m 2 s and for inlet subcooling levels ranging from 2 to 31 °C.

Heat transfer during transient cooling of high temperature surface with an impinging jet

Abstract: An experimental study of transient boiling heat transfer during a cooling of a hot cylindrical block with an impinging water jet has been made at atmospheric pressure. The experimental data were taken for the following conditions: a degree of subcooling of ΔT sub = 20–80 K, a jet velocity of u j = 5–15 m/s, a nozzle diameter of d j = 2 mm and three materials of copper, brass and carbon steel. The block was initially and uniformly heated to about 250 °C and the transient temperatures in the block were measured at eight locations in r-direction at two different depths from the surface during the cooling of hot block. The surface heat flux distribution with time was evaluated using a numerical analysis of 2-D heat conduction. Behavior of the wetting front, which is extending the nucleate boiling region outward, is observed with a high-speed video camera. A position of wetting region is measured and it is correlated well with a power function of time. The changes in estimated heat flux and temperature were compared with the position of wetting region to clarify the effects of subcooling, jet velocity and thermal properties of block on the transient cooling.

Flow of Refrigerant 134a Through Orifice Tubes

Abstract: In this paper, experimental data and a model for predicting refrigerant flow through orifice tubes used as expansion devices in automotive and other air conditioning systems are presented. The results are analyzed for an extensive set (933 data points) of measurements of mass flow rate of 1,1,1,2,-tetrafluoroethane (CH2FCF3, better known as refrigerant R-134a) through orifice tubes of different diameters and lengths, with and without inlet and outlet screens, over a wide range of operating conditions, and in the range of inlet qualities from 0 to 1 and subcooling up to 40°C. The mass flow rate through orifice tubes was found to be a strong function of inlet pressure, inlet subcooling, and diameter, but a relatively weak function of length. The semi-empirical model is developed using the data set and is applicable over the complete range of operating conditions covering transients and very high vapor quality inlets that are common in automotive air conditioning applications.

Condensation heat transfer of a pure fluid and binary mixture outside a bundle of smooth horizontal tubes. Comparison of experimental results and a classical model

Abstract: The condensation of pure HFC134a and different zeotropic mixtures with pure HFC134a and HFC23 on the outside of a bundle of smooth tubes was studied. The local heat transfer coefficient for each row was experimentally determined using a test section composed by a 13×3 staggered bundle of smooth copper tubes, measuring cooling water temperature in the inlet and the outlet of each tube, and measuring the vapour temperature along the bundle. All data were taken at the inlet vapour temperature of 40°C with a wall subcooling ranging from 4 to 26 K. The heat flux was varied from 5 to 30 kW/m2 and the cooling water flow rate from 120 to 300 l/h for each tube. The visualisation of the HFC134a condensate flow by means of transparent glass tubes reveals specific flow patterns and explains the difference between the measured values of the heat transfer coefficient and the calculated values from Nusselt's theory. On the other hand, the experimental heat transfer data with the binary mixtures HFC23-HFC134a show the important effects of temperature glide and the strong decrease of the heat transfer coefficient in comparison with the pure HFC134a data. The measured values with the different zeotropic mixtures were compared with the data calculated with the classical condensation model based on the equilibrium model. An improvement of this model is proposed.

Study of Subcooled Film Boiling on a Horizontal Disc: Part I—Analysis

Abstract: In this work subcooled film boiling on a horizontal disc was studied analytically/ numerically. Linearized stability analysis of a vapor film underlying a pool of heavier liquid was performed in three-dimensional, cylindrical coordinates. From the analysis the dominant wavelength and configuration of vapor releasing nodes was identified. Complete numerical simulation of the nonlinearly evolving interface have been carried out in axisymmetric coordinates. Finite difference method was used to simultaneously solve the equations governing conservation of mass, momentum, and energy in the vapor and liquid phases. The equations for the two phases were coupled through the matching of normal and tangential stresses and continuity of mass and energy at the interface. Second order projection method was employed for decoupling velocities from pressure. Numerical grid generation method was utilized to construct a grid system which was aligned with the interface. From the simulations the shape of the nonlinearly evolving interface, the growth rate of the interface, the flow and temperature fields in the vapor and liquid, and rate of heat transfer from the wall and into the subcooled liquid have been determined.

Study of Subcooled Film Boiling on a Horizontal Disc:1 Part 2—Experiments

Abstract: Experiments were performed to study subcooled film boiling of performance liquid PF-5060 (made by 3-M Company) on a horizontal copper disc. The experiments were performed for two regimes of film boiling involving departing vapor bubbles (low subcooling) and nondeparting vapor bubbles (high subcooling). By employing high speed digital camera, data were obtained for temporal variation of bubble height, bubble shape and bubble growth rate over one cycle. Heat flux data were deduced from temperatures measured with thermocouples embedded in the solid. The results from the numerical model are compared with experimental data and are found to be in general agreement. Particle Tracking Velocimetry (PTV) experiments were performed for a configuration of non-departing vapor bubbles to study the flow field in the liquid phase. The PTV experiments point to the existence of natural convection flow in the liquid phase and is in qualitative agreement with the predictions available in the literature.

Pool Boiling of FC-72 and HFE-7100

Abstract: This work reported the boiling characteristics of FC-72 and HFE-7100 at atmospheric pressure and at a liquid subcooling of 0-20 K. The FC-72 exhibits a more efficient nucleate boiling mode and a higher critical heat flux (CHF) than the HFE-7100. For film boiling mode, HFE-7100 becomes more efficient.

Experimental study on melting in a rectangular enclosure heated below with discrete heat sources

Abstract: The melting process of n-octadecane in a rectangular cavity with three discrete protruding heat sources on its bottom surface was studied experimentally. It was observed that the experimental process, for the geometric arrangement in this paper, is neither a fixed melting nor a contact melting, but one in which fixed melting and contact melting take place alternatively. The effects of Stefan number, initial subcooling and aspect ratio on the melting process are reported. The larger the Stefan number, the more frequently the contact melting may occur, so does the aspect ratio. The initial subcooling plays a role only in early stage. As the melting process proceeds, its effect on the melting process becomes less.

Advanced, energy efficient air conditioning, dehumidification and reheat method and apparatus

Abstract: An air conditioner controller comprises a supply air duct with an air inlet, an air outlet and a blower. An evaporator is within the duct adjacent to the inlet. A subcooling heat exchanger is within the duct between the evaporator and the outlet. A liquid line feeds coolant from a condenser. The liquid line has a primary outlet coupled to the subcooling heat exchanger and a secondary outlet coupled to a first valve. A coupling line feeds coolant from the subcooling heat exchanger to an evaporator. The coupling line has a second valve adjacent to the subcooling heat exchanger and an intermediate line. A humidistat, under the control of an operator, controls the first and second valve to thereby vary the operating parameters.

Combined refrigeration system with a liquid pre-cooling heat exchanger

Abstract: A compressor-pump unit for use in a vapor-compression refrigeration system is provided. The compressor-pump unit comprises a driving device including a rotatable shaft. A compressor is coupled with a first portion of the shaft for compressing gaseous refrigerant within the vapor-compression refrigeration system. A liquid pump is coupled with a second portion of the shaft for receiving liquid refrigerant having a first pressure and for discharging the received liquid refrigerant at a second pressure with the second pressure being higher than the first pressure by a predetermined amount such that the discharged liquid refrigerant is subcooled. A pre-cooling circuit is connected to the liquid pump with the pre-cooling circuit being exposed to the gaseous refrigerant whereby the gaseous refrigerant absorbs heat from the liquid refrigerant, prior to the liquid refrigerant entering the liquid pump.

Subcooling-type condenser

Abstract: A subcooling-type condenser includes a refrigerant condensation core and a subcooling core for supercooling refrigerant condensed by the refrigerant condensation core A header portion corresponding to an entrance portion of the subcooling core is formed as a liquid refrigerant storage portion, and a capacity of the header Vh is set within a range of 100 cc≦Vh≦250 cc Thus, subcooling-type condenser having a desired re-liquefaction function without using a separately formed liquid tank may be achieved, thereby reducing the condenser's size and cost

Multicomponent refrigeration fluid refrigeration system with auxiliary ammonia cascade circuit

Abstract: A refrigeration system wherein an ammonia refrigerant fluid in a cascade refrigeration circuit provides auxiliary refrigeration to a multicomponent refrigerant fluid which passes from an autorefrigerator to provide refrigeration to a heat load.

An experimental study of liquid jet impingement cooling of electronic components with and without boiling

Abstract: With the ever increasing heat fluxes in electronic components, enhanced cooling technology is required. This paper describes a study of heat transfer from single-phase and two-phase boiling free jet impingement. Experiments were carried out on a free jet impinging on the surface of a 2 mm and 3 mm thick film resistor. Different jet velocities and subcooling were studied. Boiling curves were obtained from the experiments. It was found that higher velocities led to higher heat transfer coefficients in single-phase heat transfer while jet diameter had very little effect on single-phase heat transfer. However, higher velocities and larger jet diameters normally led to higher heat transfer coefficients in boiling heat transfer. Active nucleation sites distribution coming from probabilistic effects and surface conditions may have a strong influence on the boiling heat transfer. Subcooling had an apparent effect on boiling heat transfer by changing flow and thermal patterns. The critical heat flux (CHF) was found to increase with increasing jet velocity, jet diameter and jet inlet subcooling. The transient CHF value under large increment of the heating power was lower than steady-state CHF, which was possibly caused by heterogeneous spontaneous nucleation (HSN). Correlation on steady-state CHF was consistent with the experimental results.

Heat Transfer in Direct Contact Condensation of Steam to Subcooled Water Spray

Abstract: The condensation heat transfer of saturated steam to a hollow-cone spray of subcooled water was investigated experimentally and analytically. The spray water temperature rose more steeply in flow direction than those in the previous studies, because of the use of smaller thermocouple which was capable of measuring the temperature in a thin water sheet and water droplets more accurately. The result of the condensation heat transfer coefficient suggested the breakup of the water sheet into droplets. A pure conduction model underpredicted the heat transfer in the sheet region significantly, which was better predicted by considering turbulence in the sheet. The heat transfer in the droplet region was well estimated by considering internal circulation and mixing inside the droplets.