Thermo-chemical characteristics of R134a flow boiling in helically coiled tubes at low mass flux and low pressure

- Chen, Chang-Nian, Han, Ji-Tian, Jen, Tien-Chien, Shao, Li

**Authors:**Chen, Chang-Nian , Han, Ji-Tian , Jen, Tien-Chien , Shao, Li**Date:**2011**Subjects:**Helically coiled tubes , Heat transfer , Low mass flux , Low pressure**Type:**Article**Identifier:**uj:5282 , http://hdl.handle.net/10210/14951**Description:**The characteristics of R134a heat transfer coefficients and wall temperature distribution were investigated under low mass flux and low pressure conditions in a helically coiled tube with heated length of 7070mm, outer diameter of 10mm, inner diameter of 7.6mm, coil diameter of 300mm and helical pitch of 40mm. System pressures, mass fluxes and inlet qualities range from 0.20 to 0.75 MPa, 50 to 260 kg/m2 s and −0.18 to 0.40, respectively. It was found that the wall temperatures in descending segments of coiled tube were higher than those of climbing ones, while the heat transfer coefficients varied inversely. Around the section circumference, the outside temperature was lower than the inside one; this is more apparent at very low mass flux and pressure conditions. The heat transfer coefficient increases with increasing mass flux, vapor quality and heat flux. However, the pressure has an indeterminate effect. New heat transfer coefficient correlations for current conditions were developed comparing with existing correlations.**Full Text:**

**Authors:**Chen, Chang-Nian , Han, Ji-Tian , Jen, Tien-Chien , Shao, Li**Date:**2011**Subjects:**Helically coiled tubes , Heat transfer , Low mass flux , Low pressure**Type:**Article**Identifier:**uj:5282 , http://hdl.handle.net/10210/14951**Description:**The characteristics of R134a heat transfer coefficients and wall temperature distribution were investigated under low mass flux and low pressure conditions in a helically coiled tube with heated length of 7070mm, outer diameter of 10mm, inner diameter of 7.6mm, coil diameter of 300mm and helical pitch of 40mm. System pressures, mass fluxes and inlet qualities range from 0.20 to 0.75 MPa, 50 to 260 kg/m2 s and −0.18 to 0.40, respectively. It was found that the wall temperatures in descending segments of coiled tube were higher than those of climbing ones, while the heat transfer coefficients varied inversely. Around the section circumference, the outside temperature was lower than the inside one; this is more apparent at very low mass flux and pressure conditions. The heat transfer coefficient increases with increasing mass flux, vapor quality and heat flux. However, the pressure has an indeterminate effect. New heat transfer coefficient correlations for current conditions were developed comparing with existing correlations.**Full Text:**

Fluid-to-fluid modelling of two-phase flow critical heat flux in horizontal helically coiled tubes

- Chen, Chang-Nian, Han, Ji-Tian, Jen, Tien-Chien, Shao, Li, Chen, Wen-wen

**Authors:**Chen, Chang-Nian , Han, Ji-Tian , Jen, Tien-Chien , Shao, Li , Chen, Wen-wen**Date:**2011**Subjects:**Critical heat flux , Fluid flow , Helically coiled tubes**Type:**Article**Identifier:**uj:5287 , http://hdl.handle.net/10210/14956**Description:**The new similarity laws for fluid-to-fluid modeling of two-phase flow critical heat flux (CHF) in horizontal helically coiled tubes were derived based on the dimensional analysis and similarity theory considering the effect of the geometrical parameters on CHF. A generalized factor Dn was introduced to the new similarity laws, and all the new dimensionless numbers were derived from the classical theorem of Buckingham for dimensional analysis. The obtained dimensionless parameter sets were a reasonable extension to Ahmad’s compensated distortion model, which may be considered as a special case of the new dimensionless parameter sets when the variable n is equal to unity. Based on the experimental data, the specific similarity numbers were determined for CHF phenomena in horizontal helically coiled tubes. A new equivalent characteristic parameter De-helix was developed, which could reflect the influence of complex flow channels on the occurrence of CHF. The equivalent characteristic parameter consists of the essential geometrical parameters of tubes and the fluid thermophysical properties. The new fluid-tofluid modeling methods were proposed for CHF of R134a-water in horizontal helically coiled tubes, which could be used readily to derive the CHF data of water through the CHF data of R134a at the corresponding experimental conditions.**Full Text:**

**Authors:**Chen, Chang-Nian , Han, Ji-Tian , Jen, Tien-Chien , Shao, Li , Chen, Wen-wen**Date:**2011**Subjects:**Critical heat flux , Fluid flow , Helically coiled tubes**Type:**Article**Identifier:**uj:5287 , http://hdl.handle.net/10210/14956**Description:**The new similarity laws for fluid-to-fluid modeling of two-phase flow critical heat flux (CHF) in horizontal helically coiled tubes were derived based on the dimensional analysis and similarity theory considering the effect of the geometrical parameters on CHF. A generalized factor Dn was introduced to the new similarity laws, and all the new dimensionless numbers were derived from the classical theorem of Buckingham for dimensional analysis. The obtained dimensionless parameter sets were a reasonable extension to Ahmad’s compensated distortion model, which may be considered as a special case of the new dimensionless parameter sets when the variable n is equal to unity. Based on the experimental data, the specific similarity numbers were determined for CHF phenomena in horizontal helically coiled tubes. A new equivalent characteristic parameter De-helix was developed, which could reflect the influence of complex flow channels on the occurrence of CHF. The equivalent characteristic parameter consists of the essential geometrical parameters of tubes and the fluid thermophysical properties. The new fluid-tofluid modeling methods were proposed for CHF of R134a-water in horizontal helically coiled tubes, which could be used readily to derive the CHF data of water through the CHF data of R134a at the corresponding experimental conditions.**Full Text:**

A heat transfer correlation of flow boiling in micro-finned helically coiled tube

- Cui, Wenzhi, Li, Longjian, Xin, Ming-dao, Jen, Tien-Chien, Chen, Qinghua, Liao, Quan

**Authors:**Cui, Wenzhi , Li, Longjian , Xin, Ming-dao , Jen, Tien-Chien , Chen, Qinghua , Liao, Quan**Date:**2006**Subjects:**Convective boiling , Heat transfer , Helically coiled tubes , Nucleate boiling**Type:**Article**Identifier:**uj:5275 , http://hdl.handle.net/10210/14944**Description:**Two main mechanisms, nucleate boiling and convective boiling, are widely accepted for in-tube flow boiling. Since the active nuclei on the heated wall are dominant for nucleate boiling and flow pattern governs the convective boiling, the heat transfer coefficient is strongly influenced by the wall heat flux, mass flux and vapor quality, respectively. In practical industrial applications, for example, the evaporators in refrigeration, forced convective evaporation is the dominant process and high heat transfer efficiency can be obtained under smaller temperature difference between wall and liquid. Therefore, it is of importance to develop a correlation of convective boiling heat transfer with a good accuracy. In this paper, a new kind of micro-finned helically coiled tube was developed and the flow boiling heat transfer characteristics were experimentally studied with R134a. Based on the analysis of the mechanisms of flow boiling, heat transfer correlations of the specific micro-finned helically coiled tubes are obtained.**Full Text:**

**Authors:**Cui, Wenzhi , Li, Longjian , Xin, Ming-dao , Jen, Tien-Chien , Chen, Qinghua , Liao, Quan**Date:**2006**Subjects:**Convective boiling , Heat transfer , Helically coiled tubes , Nucleate boiling**Type:**Article**Identifier:**uj:5275 , http://hdl.handle.net/10210/14944**Description:**Two main mechanisms, nucleate boiling and convective boiling, are widely accepted for in-tube flow boiling. Since the active nuclei on the heated wall are dominant for nucleate boiling and flow pattern governs the convective boiling, the heat transfer coefficient is strongly influenced by the wall heat flux, mass flux and vapor quality, respectively. In practical industrial applications, for example, the evaporators in refrigeration, forced convective evaporation is the dominant process and high heat transfer efficiency can be obtained under smaller temperature difference between wall and liquid. Therefore, it is of importance to develop a correlation of convective boiling heat transfer with a good accuracy. In this paper, a new kind of micro-finned helically coiled tube was developed and the flow boiling heat transfer characteristics were experimentally studied with R134a. Based on the analysis of the mechanisms of flow boiling, heat transfer correlations of the specific micro-finned helically coiled tubes are obtained.**Full Text:**

Experimental study on critical heat flux characteristics of R134a flow boiling in horizontal helically-coiled tubes

- Chen, Chang-Nian, Han, Ji-Tian, Jen, Tien-Chien, Shao, Li, Chen, Wen-wen

**Authors:**Chen, Chang-Nian , Han, Ji-Tian , Jen, Tien-Chien , Shao, Li , Chen, Wen-wen**Date:**2011**Subjects:**Critical heat flux , Helically coiled tubes , Heat transfer**Type:**Article**Identifier:**uj:5284 , http://hdl.handle.net/10210/14953**Description:**Critical heat flux (CHF) experiments were performed to study the R134a CHF characteristics in horizontal helically-coiled tubes. The stainless steel test sections were heated uniformly, with tube inner diameters of 3.8e11 mm, coil diameters of 135e370 mm, helical pitches of 40e105 mm and heated lengths of 0.85e7.54 m. The experimental conditions are pressures of 0.30e1.10 MPa, mass fluxes of 60e480 kg m 2 s 1, inlet qualities of 0.32e0.36 and heat fluxes of 6.0 103e9.0 104Wm 2. It was found that the wall temperatures jumped abruptly once the CHF occurred. The CHF values decrease with increasing heated lengths, coil diameters and inner diameters, but the DNB (departure from nucleate boiling) CHF seems independent when length-to-diameter L/di> 200. The coil-to-diameter ratios are more important than length-to-diameter ratios for CHF in helically-coiled tubes, while the helical pitches have little effect on CHF. The CHF value increases greatly with increasing mass flux and decreases smoothly with increasing pressure. It decreases nearly linearly with increasing inlet and critical qualities, but it varies more acutely at xcr< 0.5 than higher critical qualities. New correlations for R134a flow boiling CHF in horizontal helically-coiled tubes were developed for applications.**Full Text:**

**Authors:**Chen, Chang-Nian , Han, Ji-Tian , Jen, Tien-Chien , Shao, Li , Chen, Wen-wen**Date:**2011**Subjects:**Critical heat flux , Helically coiled tubes , Heat transfer**Type:**Article**Identifier:**uj:5284 , http://hdl.handle.net/10210/14953**Description:**Critical heat flux (CHF) experiments were performed to study the R134a CHF characteristics in horizontal helically-coiled tubes. The stainless steel test sections were heated uniformly, with tube inner diameters of 3.8e11 mm, coil diameters of 135e370 mm, helical pitches of 40e105 mm and heated lengths of 0.85e7.54 m. The experimental conditions are pressures of 0.30e1.10 MPa, mass fluxes of 60e480 kg m 2 s 1, inlet qualities of 0.32e0.36 and heat fluxes of 6.0 103e9.0 104Wm 2. It was found that the wall temperatures jumped abruptly once the CHF occurred. The CHF values decrease with increasing heated lengths, coil diameters and inner diameters, but the DNB (departure from nucleate boiling) CHF seems independent when length-to-diameter L/di> 200. The coil-to-diameter ratios are more important than length-to-diameter ratios for CHF in helically-coiled tubes, while the helical pitches have little effect on CHF. The CHF value increases greatly with increasing mass flux and decreases smoothly with increasing pressure. It decreases nearly linearly with increasing inlet and critical qualities, but it varies more acutely at xcr< 0.5 than higher critical qualities. New correlations for R134a flow boiling CHF in horizontal helically-coiled tubes were developed for applications.**Full Text:**

An experiment study of flow pattern and pressure drop for flow boiling inside microfinned helically coiled tube

- Cui, Wenzhi, Li, Longjian, Xin, Ming-dao, Jen, Tien-Chien, Liao, Quan, Chen, Qinghua

**Authors:**Cui, Wenzhi , Li, Longjian , Xin, Ming-dao , Jen, Tien-Chien , Liao, Quan , Chen, Qinghua**Date:**2008**Subjects:**Flow boiling , Pressure drop , Helically coiled tubes , Microfin tubes , Flow patterns , Finned tubes**Type:**Article**Identifier:**uj:5278 , http://hdl.handle.net/10210/14947**Description:**In this paper, flow patterns and their transitions for refrigerant R134a boiling in a microfinned helically coiled tube are experimentally observed and analyzed. All the flow patterns occurred in the test can be divided into three dominant regimes, i.e., stratified-wavy flow, intermittent flow and annular flow. Experimental data are plotted in two kinds of flow maps, i.e., Taitel and Dukler flow map and mass flux versus vapor quality flow map. The transitions between various flow regimes and the differences from that in smooth straight tube have also been discussed. Martinelli parameter can be used to indicate the transition from intermittent flow to annular flow. The transition from stratified-wavy flow to annular or intermittent flow is identified in the vapor quality versus mass flux flow map. The flow regime is always in stratified-wavy flow for a mass flux less than 100 kg/m2 s. The two-phase frictional pressure drop characteristics in the test tube are also experimentally studied. The two-phase frictional multiplier data can be well correlated by Lockhart–Martinelli parameter. Considering the corresponding flow regimes, i.e., stratified and annular flow, two frictional pressure drop correlations are proposed, and show a good agreement with the respective experimental data.**Full Text:**

**Authors:**Cui, Wenzhi , Li, Longjian , Xin, Ming-dao , Jen, Tien-Chien , Liao, Quan , Chen, Qinghua**Date:**2008**Subjects:**Flow boiling , Pressure drop , Helically coiled tubes , Microfin tubes , Flow patterns , Finned tubes**Type:**Article**Identifier:**uj:5278 , http://hdl.handle.net/10210/14947**Description:**In this paper, flow patterns and their transitions for refrigerant R134a boiling in a microfinned helically coiled tube are experimentally observed and analyzed. All the flow patterns occurred in the test can be divided into three dominant regimes, i.e., stratified-wavy flow, intermittent flow and annular flow. Experimental data are plotted in two kinds of flow maps, i.e., Taitel and Dukler flow map and mass flux versus vapor quality flow map. The transitions between various flow regimes and the differences from that in smooth straight tube have also been discussed. Martinelli parameter can be used to indicate the transition from intermittent flow to annular flow. The transition from stratified-wavy flow to annular or intermittent flow is identified in the vapor quality versus mass flux flow map. The flow regime is always in stratified-wavy flow for a mass flux less than 100 kg/m2 s. The two-phase frictional pressure drop characteristics in the test tube are also experimentally studied. The two-phase frictional multiplier data can be well correlated by Lockhart–Martinelli parameter. Considering the corresponding flow regimes, i.e., stratified and annular flow, two frictional pressure drop correlations are proposed, and show a good agreement with the respective experimental data.**Full Text:**

Dry-out CHF correlation for R134a flow boiling in a horizontal helically-coiled tube

- Chen, C.-N., Han, J.-T., Jen, T.-C., Shao, L.

**Authors:**Chen, C.-N. , Han, J.-T. , Jen, T.-C. , Shao, L.**Date:**2011**Subjects:**Critical heat flux , Helically coiled tubes , Heat transfer**Type:**Article**Identifier:**uj:5286 , http://hdl.handle.net/10210/14955**Description:**An experimental study was carried out to investigate the R134a dry-out critical heat flux (CHF) characteristics in a horizontal helically-coiled tube. The test section was heated uniformly by DC high-power source, and its geometrical parameters are the outer diameter of 10 mm, inner diameter of 8.4 mm, coil diameter of 300 mm, helical pitch of 75 mm and valid heated length of 1.89 m. The experimental parameters are the outlet pressures of 0.30–0.95 MPa, mass fluxes of 60–500 kg m 2 s 1, inlet qualities of 0.36–0.35 and heat fluxes of 7.0 103–5.0 104 Wm 2. A method based on Agilent BenchLink Data Logger Pro was developed to determine the occurrence of CHF with a total of 68 T-type thermocouples (0.2 mm) set along the tube for accurate temperature measurement. The characteristics of wall temperatures and the parametric effect on dry-out CHF showed that temperature would jump abruptly at the point of CHF, which usually started to form at the front and offside (270 and 90 ) of the outlet crosssection. The CHF values decrease nearly linearly with increasing inlet qualities, while they decrease more acutely with increasing critical qualities, especially under larger mass flux conditions. The mass flux has a positive effect on CHF enhancement, but the pressure has negative one. A new dimensionless correlation was developed to estimate dry-out CHF of R134a flow boiling in horizontal helically-coiled tubes under current experimental conditions and compared to calculated results from Bowring and Shah correlations.**Full Text:**

**Authors:**Chen, C.-N. , Han, J.-T. , Jen, T.-C. , Shao, L.**Date:**2011**Subjects:**Critical heat flux , Helically coiled tubes , Heat transfer**Type:**Article**Identifier:**uj:5286 , http://hdl.handle.net/10210/14955**Description:**An experimental study was carried out to investigate the R134a dry-out critical heat flux (CHF) characteristics in a horizontal helically-coiled tube. The test section was heated uniformly by DC high-power source, and its geometrical parameters are the outer diameter of 10 mm, inner diameter of 8.4 mm, coil diameter of 300 mm, helical pitch of 75 mm and valid heated length of 1.89 m. The experimental parameters are the outlet pressures of 0.30–0.95 MPa, mass fluxes of 60–500 kg m 2 s 1, inlet qualities of 0.36–0.35 and heat fluxes of 7.0 103–5.0 104 Wm 2. A method based on Agilent BenchLink Data Logger Pro was developed to determine the occurrence of CHF with a total of 68 T-type thermocouples (0.2 mm) set along the tube for accurate temperature measurement. The characteristics of wall temperatures and the parametric effect on dry-out CHF showed that temperature would jump abruptly at the point of CHF, which usually started to form at the front and offside (270 and 90 ) of the outlet crosssection. The CHF values decrease nearly linearly with increasing inlet qualities, while they decrease more acutely with increasing critical qualities, especially under larger mass flux conditions. The mass flux has a positive effect on CHF enhancement, but the pressure has negative one. A new dimensionless correlation was developed to estimate dry-out CHF of R134a flow boiling in horizontal helically-coiled tubes under current experimental conditions and compared to calculated results from Bowring and Shah correlations.**Full Text:**

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