Top PDF A new correlation for heat transfer during flow boiling

A new correlation for heat transfer during flow boiling

A new correlation for heat transfer during flow boiling

This Thesis is brought to you for free and open access by the Thesis/Dissertation Collections at RIT Scholar Works. It has been accepted for inclusion in Theses by an authorized administrator of RIT Scholar Works. For more information, please contact ritscholarworks@rit.edu. Recommended Citation Thakur, Bhabesh K., "A new correlation for heat transfer during flow boiling" (1981). Thesis. Rochester Institute of Technology.

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Heat transfer correlation for flow boiling in small to micro tubes

Heat transfer correlation for flow boiling in small to micro tubes

compared to the other models we examined and can be used in a lot of cases due its ease of application. The performance of most correlations changed with the change of diameter and heated length. This may be due to the fact that most microscale correla- tions were developed based on channels of short lengths. If the length is too short, the applied heat flux must be much higher for the same exit quality compared to long channels. Accordingly, there is a possibility for the nucleate boiling mechanism to domi- nate, i.e. the local heat transfer coefficient does not vary with local quality. Therefore, it is expected that these correlations perform poorly as the heated length increases, i.e. as h increases with x to- wards the channel exit. A new correlation of the Chen type was proposed in this paper, which is more general especially for refrig- erants and can predict local and hence average heat transfer coef- ficient values. The new correlation predicts the increasing trend of h vs. x that was observed in micro tubes. The correlation predicted 92% of all data within the ±30% error bands at a MAE value of 14.3%. The new correlation predicted satisfactorily the data for the three different lengths available for the 1.1 mm dia. tube. How- ever, the effect of heated length need to be examined further and included in a future version probably as a non-dimensional term (L/D).
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Flow boiling of R452A: Heat transfer data, dry-out characteristics and a correlation

Flow boiling of R452A: Heat transfer data, dry-out characteristics and a correlation

N.B.: This is the PREPRINT (submitted) version of this article. The final, published version of the article can be found at: https://doi.org/10.1016/j.expthermflusci.2019.04.006 Abstract This paper presents an experimental investigation on two-phase heat transfer and dry-out occurrence for refrigerant R452A in a single horizontal circular stainless-steel tube having an internal diameter of 6.0 mm. The effects of mass flux (from 150 to 600 kg/m 2 s), saturation (bubble) temperature (from 23 to 55 °C) and heat flux (from 10 to 65 kW/m 2 ) are investigated and discussed. Heat transfer coefficient and dry-out vapor quality data are then compared to R404A results in the same operating conditions, observing that the nucleate boiling contribution of the new blend is penalized by its very high glide temperature during evaporation. The assessment of some dry-out and flow boiling heat transfer coefficient prediction methods is finally carried-out and a correction factor on the nucleate boiling term is proposed to take into account the negative effect of the glide temperature difference on the mass diffusion in the liquid. By implementing this modification on two chosen asymptotic models, the statistical error analysis is considerably improved.
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Influence of wettability on boiling heat transfer and critical heat flux in vertical flow boiling

Influence of wettability on boiling heat transfer and critical heat flux in vertical flow boiling

12 1.2 Objectives From a review of literature, although some effort has been made in pool conditions, few studies have systematically analyzed the effect of wettability on heat transfer and CHF in flow boiling. The objectives of this thesis are to meet the data needs for improved understanding of the influence of surface wettability on heat transfer and CHF in flow boiling, and evaluate the prediction capability of existing CHF models. This is accomplished by characterizing the effect of various system parameters on the boiling properties and CHF values for a hydrophilic surface and comparing them to the effects on a hydrophobic surface. The hydrophilic surface is a polished copper surface studied over a range of pressure, mass flux, and inlet subcooling values up to CHF.
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A CRITICAL REVIEW OF RECENT INVESTIGATIONS ON FLOW PATTERN AND HEAT TRANSFER DURING FLOW BOILING IN MICRO-CHANNELS

A CRITICAL REVIEW OF RECENT INVESTIGATIONS ON FLOW PATTERN AND HEAT TRANSFER DURING FLOW BOILING IN MICRO-CHANNELS

Saisorn et al. (2010) p erformed flow visualization study for R- 134a refrigerant during flow boiling in a circular channel having a diameter of 1.75 mm. Slug flow, throat-annular flow, churn flow, annular flow and annular-rivulet flow were observed and found t o influence the flow boiling heat transfer process as seen in Fig. 2. Slug flow appeared with the lowest heat transfer coefficient in comparison to the other flow regimes. Annular-rivulet flow showed a relatively high heat transfer coefficient but a local dry-out region was observed at high vapour qualities, which has been undesirable for a thermal design approach dealing with a cooling system implemented with small channels. Moderate values of he at transfer coefficient were given by throat-annular flow, churn flow and annular flow which might be good choices for t he development of t he micro-scale devices. Besides, their flow pattern data were compared with the transition lines by Triplett et al. (1999) for t wo-phase air-water flow through a 1.45 mm diameter channel. In general, the comparisons showed inconsistencies between the flow pattern map established from two-phase gas-liquid flow and that from phase-change process. Such inconsistencies were also reported by Yang and Shieh (2001) and Martin-Callizo et al. (2010). Yang and Shieh (2001) performed flow visualization with air-water mixture and refrigerant R-134a, and the comparison between such two cases were discussed. Martin- Callizo et al. (2010) c onducted the visualization of R -134a during flow boiling in a tube with a diameter of 1.33 m m. Their test section was made from a quartz glass tube coated externally by Indium Tin Oxide (ITO) which was served as the resistive coating over which a potential difference generated by a DC power supply was applied. Their flow pattern data were also compared with the transition lines by Triplett et al. (1999), i ndicating that the agreement was not satisfactory. However, two-phase gas-liquid flow phenomena tend to be compatible with flow mechanisms based on phase-change process in different aspects. In m icro-channels, for instance, Saisorn and Wongwises (2010) re ported the fair agreement between their gas- liquid flow pattern data and the transition lines of G arimella et al. (2002) for condensation flow.
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Liquid and Flow Boiling Heat Transfer Inside a Copper Foam

Liquid and Flow Boiling Heat Transfer Inside a Copper Foam

Considering the data reduction, in the case of flow boiling heat transfer measurements, the vapour quality at the inlet of the test section depends on the refrigerant conditions at the i[r]

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Heat transfer coefficient for flow boiling in an annular mini gap

Heat transfer coefficient for flow boiling in an annular mini gap

In the present paper, one- and two-dimensional mathematical models are proposed to describe the stationary heat transfer in flow boiling of cooling liquid in the cylindrical annular gap. A set of experimental data governs the form of energy equations in the cylindrical coordinates and the boundary conditions. The two- dimensional model is formulated to minimize the number of experimentally determined constants. In addition to the fixed thermal-flow parameters, this model uses two quantities determined experimentally: the surface temperature of the thermal conductive filler layer and void fraction. The method for determining the fluid temperature distribution depends on the flow type. The
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R134a flow boiling heat transfer in small diameter tubes

R134a flow boiling heat transfer in small diameter tubes

ing in a horizontal small circular pipe of 2.0 mm inside diameter. They noted that the boiling heat transfer coefficient was higher at a higher imposed wall heat flux except in the high vapour quality region, and also, the boiling heat transfer coefficient was higher at a higher mass flux and saturation temperature when the imposed heat flux was low. Vertical flow boiling of R134a in small multi-channels was investigated by Agostini and Bontemps (2004). Their experimental results indicated that heat transfer rates were greater than that reported in the previous litera- ture for conventional tubes, while dry-out occurred at low qualities. However, from their results, it was very difficult to conclude which regime was dominant, nucleation or forced con- vection.
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Experimental study of flow boiling heat transfer and critical heat flux in microchannels

Experimental study of flow boiling heat transfer and critical heat flux in microchannels

The effect of pressure drop elements (PDEs) on flow boiling heat transfer is also presented in this section. All tests in this section are conducted with microchannels in the horizontal orientation with water as the working liquid. The results from the case without PDEs are compared to those with 6.1% PDEs at the inlet of each channel. The latter case uses manifold which incorporates inlet openings of 127 microns diameter at the inlet to each channel, giving an open area that is 6.1% of the cross sectional area of a 1054 x 197 µm 2 microchannel. These pressure restrictors are expected to reduce the backflow by forcing an expanding vapor bubble in the downstream direction and not allowing the liquid-vapor mixture to enter the inlet manifold. One example of flow reversal using a constant mass flux of 212.8 kg/m 2 s is depicted in Fig. 10.2. Using water as the working liquid, the sequence of frames in Fig.
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Impact of selected thermal and flow parameters on flow boiling heat transfer in a minichannel

Impact of selected thermal and flow parameters on flow boiling heat transfer in a minichannel

Abstract. This paper reports an impact of selected thermal and flow parameters i.e., mass flux and inlet pressure on flow boiling heat transfer in a minichannel. Research was carried out on the experimental set up with the test section fitted with a single, rectangular and vertically oriented minichannel 1.7 mm deep. Infrared thermography was used to determine changes in the temperature on the outer side of the heated minichannel wall in the central part of the minichannel. The heated element for HFE-649 flowing in the minichannel was a thin alloy plate, made of Haynes-230. Local values of heat transfer coefficient for stationary state conditions were calculated using a simple one-dimensional method. Analysis of the results was based on experimental series obtained for the same heat flux, various mass fluxes and average inlet pressures. The experimental results are presented as the relationship between the heat transfer coefficient and the distance along the minichannel length and boiling curves. The highest local heat transfer coefficients were obtained for the lower average inlet pressure and for the highest mass flux at lower heat flux.
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Experimental Study on Heat Transfer and Flow Characteristics in Subcooled Flow Boiling in a Microchannel

Experimental Study on Heat Transfer and Flow Characteristics in Subcooled Flow Boiling in a Microchannel

183 3.2.2 Effect of heat flux on heat transfer In both micro and macro scale, the impact of heat flux represents an essential part in treating the case of representing predominant flow boiling heat transfer mechanism(s). The nucleate boiling mechanism is assumed to be predominant when the heat transfer coefficient does not vary with vapor quality and mass flux and increases with increasing heat flux. Also, convective boiling is considered to be the dominant heat transfer mechanism when the heat transfer coefficient does not depend on heat flux and increases with increasing mass flux and vapor quality. To determine the impact of the heat flux upon the local heat transfer coefficient, obviously for the test section, the heat fluxes are divided into two groups the first group with low heat flux values. In contrast, the others are considered as moderate to high heat fluxes. Figs. 9 and 10 show the variation of the low heat flux and high heat flux on local two-phase heat transfer coefficient, respectively, at 1700 kg /m 2 s mass flux and subcooled inlet fluid temperature is 31 ˚C. In Fig.9, the heat transfer coefficient remains in a single-phase region and it increases with increasing heat flux. The reason for that is the thermal boundary layer was not fully developed. Also, Fig.9 shows that the heat transfer coefficient increased along the microchannel test part length for the fixed heat flux because of the effect wall temperature of microchannel increase in the axial direction due to the axial heat conduction effect.
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Flow boiling heat transfer of refrigerant R-134a in copper microchannel heat sink

Flow boiling heat transfer of refrigerant R-134a in copper microchannel heat sink

Abstract In this paper we present experimental data on heat transfer and pressure drop characteristics at flow boiling of refrigerant R-134a in a horizontal microchannel heat sink. The primary objective of this study is to establish experimentally how the local heat transfer coefficient and pressure drop correlate with the heat flux, mass flux and vapor quality. The copper plate of microchannel heat sink contains 21 microchannels with 335x930 m 2 cross-section. The microchannel plate and heating block were divided by the partition wall for the local heat flux measurements. Distribution of local heat transfer coefficients along the length and width of the microchannel plate were measured in the range of external heat fluxes from 50 to 500 kW/m 2 ; the mass flux was varied within 200-600 kg/m 2 s, and pressure was varied within 6-16 bar. The obvious impact of heat flux on the magnitude of heat transfer coefficient was observed. It shows that nucleate boiling is the dominant mechanism for heat transfer. The new model of flow boiling heat transfer, which accounts nucleate boiling suppression and liquid film evaporation, was proposed and verified experimentally in this paper.
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Two dimensional heat transfer problem in flow boiling in a rectangular minichannel

Two dimensional heat transfer problem in flow boiling in a rectangular minichannel

Abstract. The paper presents mathematical modelling of flow boiling heat transfer in a rectangular minichannel asymmetrically heated by a thin and one-sided enhanced foil. Both surfaces are available for observations due to the openings covered with glass sheets. Thus, changes in the colour of the plain foil surface can be registered and then processed. Plain side of the heating foil is covered with a base coat and liquid crystal paint. Observation of the opposite, enhanced surface of the minichannel allows for identification of the gas- liquid two-phase flow patterns and vapour quality. A two-dimensional mathematical model of heat transfer in three subsequent layers (sheet glass, heating foil, liquid) was proposed. Heat transfer in all these layers was described with the respective equations: Laplace equation, Poisson equation and energy equation, subject to boundary conditions corresponding to the observed physical process. The solutions (temperature distributions) in all three layers were obtained by Trefftz method. Additionally, the temperature of the boiling liquid was obtained by homotopy perturbation method (HPM) combined with Trefftz method. The heat transfer coefficient, derived from Robin boundary condition, was estimated in both approaches. In comparison, the results by both methods show very good agreement especially when restricted to the thermal sublayer.
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Heat transfer coefficient determination for flow boiling in vertical and horizontal minichannels

Heat transfer coefficient determination for flow boiling in vertical and horizontal minichannels

1 Introduction Boiling is an extremely efficient heat transfer process used in power generation, chemical industry and nuclear engineering. One of the relevant boiling features is the high value of the heat transfer coefficient, due to which large heat fluxes can be transported. Miniheat exchangers are used to provide higher cooling capability for new technologies. Owing to the change of state, which accompanies flow boiling in minichannels, it is possible to meet contradictory demands simultaneously, i.e. to obtain a heat flux as large as possible at small temperature difference between the heating surface and the saturated liquid and, at the same time, retain small dimensions of heat transfer systems. Review of relevant literature and the selected publications covering flow boiling heat transfer in minichannels is presented in [1,2], and having enhanced surfaces - in [3-5]. It leads to the conclusion that although much has been written recently on flow boiling heat transfer in minichannels, the observations related to the effects of various factors on boiling heat transfer in minichannels are diverse and frequently conflicting. They are usually verified experimentally for channel systems heated by smooth heating surfaces.
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Investigation into flow boiling heat transfer in a minichannel with enhanced heating surface

Investigation into flow boiling heat transfer in a minichannel with enhanced heating surface

Boiling is a very efficient heat transfer process used in power engineering, chemical engineering and nuclear engineering. Mini heat exchangers are used in the interest of providing higher cooling capability for new technologies. It means a reduction of their size and cost, for an identical power. Owing to the change of state, which accompanies flow boiling in minichannels, it is possible to meet contradictory demands simultaneously, i.e. to obtain a heat flux as large as possible at small temperature difference between the heating surface and the saturated liquid and, at the same time, retain small dimensions of heat transfer systems.
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Flow Boiling Heat Transfer Over Open Microchannels With Tapered Manifold

Flow Boiling Heat Transfer Over Open Microchannels With Tapered Manifold

18 Chapter 2: Literature Review 2.1 Previous Work Microchannel two-phase cooling is accomplished with the help of a heat sink that consists of a high conductivity material containing parallel, small diameter channels. The simplicity and ease of fabrication of the design are the key reasons behind its unprecedented popularity in the industry. Most microchannel geometries of interest possess diameters in the range of 0.1–0.6 mm. These microchannel devices are therefore very compact and lightweight, and provide high heat transfer coefficients by capitalizing upon the coolant’s latent heat content rather than the sensible heat alone (seen in single phase liquid cooling). This greatly reduces the flow rate required to dissipate the same amount of heat compared to single-phase cooling, which also helps reduce coolant inventory for the entire cooling system. Flow boiling with microchannels also provides better temperature uniformity by maintaining surface temperatures close to the coolant’s saturation temperature. However, two-phase microchannel cooling is not without shortcomings, and their implementation is hindered by the relatively limited understanding of two-phase flow in microchannels.
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Flow Boiling Heat Transfer Over Open Microchannels With Tapered Manifold

Flow Boiling Heat Transfer Over Open Microchannels With Tapered Manifold

18 Chapter 2: Literature Review 2.1 Previous Work Microchannel two-phase cooling is accomplished with the help of a heat sink that consists of a high conductivity material containing parallel, small diameter channels. The simplicity and ease of fabrication of the design are the key reasons behind its unprecedented popularity in the industry. Most microchannel geometries of interest possess diameters in the range of 0.1–0.6 mm. These microchannel devices are therefore very compact and lightweight, and provide high heat transfer coefficients by capitalizing upon the coolant’s latent heat content rather than the sensible heat alone (seen in single phase liquid cooling). This greatly reduces the flow rate required to dissipate the same amount of heat compared to single-phase cooling, which also helps reduce coolant inventory for the entire cooling system. Flow boiling with microchannels also provides better temperature uniformity by maintaining surface temperatures close to the coolant’s saturation temperature. However, two-phase microchannel cooling is not without shortcomings, and their implementation is hindered by the relatively limited understanding of two-phase flow in microchannels.
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Experimental Investigations of Flow Boiling Heat Transfer and Flow Instability in a Horizontal Microtube with an Inlet Orifice

Experimental Investigations of Flow Boiling Heat Transfer and Flow Instability in a Horizontal Microtube with an Inlet Orifice

A 2  4 (2.1) where, H is the height of microchannel; W is the width of microchannel; L is the length of microchannel; D h is the hydraulic diameter of microchannel. A large ratio of surface area to volume is achieved by reducing the hydraulic diameter to micro-scale. This assists in developing a compact and efficient design of heat exchanger. These types of systems are quiet and can be accommodated in the restricted space inside the equipment. The advantage of cooling the electronic chips by dissipating heat through the liquid flowing in the microchannels is that the heat transfer coefficient is high as it is inversely proportional to the hydraulic diameter of the channel. It is to be noted that the coolant temperature rise along the channel is very high in case of single-phase flow because all the heat generated by the electronic device is carried away by relatively small amount of liquid. Therefore, it is preferred to have a two-phase flow cooling system. Flow boiling in microchannel heat sinks offers those same attributes while providing the following important advantages over their single-phase counterparts: much higher convective heat transfer coefficients due to large latent heat during boiling; better temperature uniformity;
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Flow boiling heat transfer of a non-azeotropic mixture inside a single microchannel

Flow boiling heat transfer of a non-azeotropic mixture inside a single microchannel

Abstract This study moves from the need to study flow boiling of zeotropic mixture in microchannels. In the recent years much attention has been paid to the possible use of fluorinated propene isomers for the substitution of high-GWP refrigerants. The available HFOs (hydrofluoroolefins) cannot cover all the air- conditioning, heat pump, and refrigeration systems when used as pure fluids because their thermodynamic properties are not suitable for all operating conditions and therefore some solutions may be found using blends of refrigerants, to satisfy the demand for a wide range of working conditions. In the present paper a mixture of R1234ze(E) and R32 (0.5/0.5 by mass) has been studied. The local heat transfer coefficient during flow boiling of this mixture in a single microchannel with 0.96 mm diameter is measured at a pressure of 14 bar, which corresponds to a bubble temperature of 26.3°C. The flow boiling data taken in the present test section are discussed, with particular regard to the effect of heat flux, mass velocity and vapor quality. The heat transfer coefficients are compared against some predicting models available in the literature.
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Fundamental issues, mechanisms and models of flow boiling heat transfer in microscale channels

Fundamental issues, mechanisms and models of flow boiling heat transfer in microscale channels

saturation temperatures, mass fluxes, heat fluxes and working fluids etc. Therefore, the extrapolation of the available prediction methods to other fluids and channels do not work properly. It must be stressed here that big discrepancies among the experimental data from different independent laboratories may be caused due to different surface roughness of the test channels, channel dimension uncertainties, improper data reduction methods, flow boiling instabilities, improper designed test facility, test sections and experimental procedures. In some cases, the published results are unreasonable such as too big or too small heat transfer coefficients, some or complete wrong heat transfer behaviours and trends and correlations of various parameters and physical properties even if they have been published in journals. For instance, quite anomaly heat transfer trends are presented but they cannot be explained according to the corresponding flow boiling mechanisms in some papers although it is said that such mechanisms account for the heat transfer behaviours as detailed in a recent analysis in a comprehensive review by Ribatski et al. [7]. Furthermore, some flow boiling heat transfer correlations and models were proposed by simply regressing limited experimental data at limited test parameter ranges without considering the heat transfer mechanisms. Therefore, it is necessary to evaluate these correlations to validate their applicability before using these correlations. In fact, in most cases, such correlations do not work properly for other fluids and conditions. Therefore, it is essential to analyse and evaluate these correlations to identify further research needs in this important field.
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