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:**

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:**

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:**

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