Search

The widely used fin-and-tube heat exchangers have a low air-side heat transfer capacity. Enhancing the air-side thermo-hydraulic performance is a key point of the relevant research. Vortex generators have been proven to be one of the most promising techniques to improve the air-side heat transfer capacity of the fin-and-tube heat exchangers. Herein, this paper proposed an original sinusoidal wavy winglet type vortex generator to achieve high heat transfer performance with moderate increased pressure drop penalty. A three-dimensional numerical simulation was conducted to explore the air-side thermo-hydraulic performance with Reynolds number ranging from 1027 to 2054. The longitudinal vortex is generated in the channel so as to enhance the intensify of convective heat transfer. Especially, the wave-shaped vortex generators can produce many local small-scale secondary flows. And it is confirmed that the frontal area of the vortex generator can be effectively extended by such wavy shape and a more intensified disturbance can be caused in the air-side. Furthermore, the effects of the attack angle (α), dimensionless height (H p) and especially arrange position of the vortex generator were investigated. Additionally, the ranges of the Nusselt number ratio, friction factor ratio and surafece goodness factor in this study are 1.09–1.52, 1.09–2.31, and 1.06–1.24, respectively. Moreover, it is also very crucial to evalute the grade of energy and to explore the irreversible loss druing the heat tranfer process in view of the second law of thermodynamics. And the thermal dissipation can be reduced by up to 5.85% compared with the plain fins from the perspective of exergy analysis. [ABSTRACT FROM AUTHOR]

A modified Allam cycle (Allam-Z cycle) with a simpler system was proposed and investigated using NG (natural gas)/O 2 combustion products mixing with the circulation CO 2 as the working medium for power generation with high efficiency, zero CO 2 emission and peak load shifting. The modifications are that all the working media are pumped to high pressure by pumps instead of compressors, the cold energy of both liquid oxygen and LNG is used for degrading the cooling water for CO 2 liquefaction and a set of regenerative heat exchangers are arranged for turbine exhaust heat recovery. The influences of turbine parameters on the performances of the cycle were investigated. The comparison was performed under the conditions of condensation temperature of 30 °C, turbine inlet pressure of 30 MPa, inlet temperature of either 700 °C or 900 °C and the turbine outlet pressures of Allam-Z cycle and Allam cycle are 7.21 MPa and 4 MPa respectively. The results show that the output power efficiency and the equivalent net efficiency of the Allam-Z cycle with full CO 2 capture are 43.64% and 40.83% respectively or 50.87% and 48.05% respectively, which are 2.15% or 2.96% higher than those of the Allam cycle under the same condition. • A modified Allam cycle was studied with combustion product and CO 2 as working medium. • Cycle system is simplified with higher turbine back pressure to eliminate compressor. • The cycle integrates power generation with CO 2 capture and energy shifting. • Equivalent net efficiency is adopted considering 1/4 ASU consumed off-peak power. • Equivalent net efficiency is 48.05% with turbine inlet parameters of 30 MPa/900 °C. [ABSTRACT FROM AUTHOR]

Axial separation is a novel baffle connection configuration of circumferential overlap helical baffle heat exchangers which makes baffle incline angle seriation possible. The adjacent baffle plates are separated by sleeve tubes to form a greater pitched helix for enhancing heat transfer under required pressure drop with the least number of molds for reducing baffle manufacture cost. Numerical simulations of flow and heat transfer performances were conducted and the calculation results were verified by the experimental ones. Performance investigation was conducted on eight normal trisection helical baffle schemes with baffle incline angles from 15° to 23° and five axial separation schemes of equivalent 17–22° angles using 15° baffles. In the calculation scope the average deviations of heat transfer coefficients and pressure drops of the normal helical schemes are respectively about −1.97% and −8.11% per 1° deviation of incline angle. The axial separation schemes using 15° baffles with equivalent 21° and 22° incline angles have deviation values of heat transfer coefficient 1.21% and 1.33% lower and pressure drop 4.96% and 5.55% higher respectively than those of the normal 21° and 22° incline angle schemes. By adopting axial separation approach the seriation step of baffle incline angles can be extended to 8°. [ABSTRACT FROM AUTHOR]

• The power plant feedwater heaters and the countercurrent U-tube heat exchangers all involve half-cylindrical shell space. • Half-cylindrical space heat exchangers with ladder helical, orifice and segmental baffles were experimentally investigated. • The ladder helical baffle scheme has superior heat transfer coefficient and comprehensive index than those of the other ones. • The orifice baffle scheme has higher comprehensive index but lower heat transfer coefficient than those of segmental ones. • The average values of h o Δ p o −1/3 of ladder helical and orifice baffle schemes are about 15% higher than that of segmental one. The superheated steam cooling section and the condensate cooling section of the power plant feedwater heaters and the countercurrent U-tube heat exchangers all involve heat transfer in half-cylindrical shell space. The overall and shell-side heat transfer coefficients (h.t.cs.), pressure drop, and the comprehensive performances of some half-cylindrical space heat exchangers with different baffle structures were experimentally investigated. The investigated structures include three ladder helical baffle types [single inclined ladder helical (SH), dual inclined ladder helical (DH) and folded ladder helical (FH)], three orifice baffle types [fillet triangle orifice (F), trefoil-orifice (T) and quatrefoil-orifice (Q)], and segmental baffles (S), and all schemes are of two baffle pitches. Under the same baffle pitch and working condition, the experimental results show that the shell-side h.t.cs. of the ladder helical baffle schemes are higher than those of the orifice baffle schemes and segmental baffle ones; and the shell-side pressure drops of the orifice baffle schemes are much lower than those of the ladder helical baffle schemes and the segmental ones. The values of average comprehensive index h o Δ p o −1/3 of the ladder helical schemes FH-40, DH-40 and the orifice baffle schemes F-40 and T-40 are respectively 14.26%, 17.58%, 15.62% and 12.82% higher than that of the segmental scheme S-40. [ABSTRACT FROM AUTHOR]

• A novel helical baffle with small inclined angle and folded structure is proposed. • The new helical baffles can be created by cheap 2D laser cutting and folding. • The leakage between two folded helical baffles can be greatly decreased. • The novel electric heaters have excellent flow and heat transfer performances. • Folded helical baffles are more excellent at large equivalent angles in this work. The difficult manufacturing of helical baffles hinders the widespread application of the enhancement technology and the shortcut leakage between adjacent baffles impairs the enhancement effects. To solve these two issues, the promising anti-leakage and easy-to-manufacture helical baffles with small inclined angle and folded structure are proposed, which can be produced by 2D laser cutting and simple mechanical folding. The influence of different helical baffle configurations, including three normal helical schemes, three axial separation helical schemes, three novel folded axial separation helical baffle schemes and a segmental scheme on the performance of electric heaters are numerically investigated. The equivalent angles of helical schemes adopted in this work are 15°, 20° and 25°, and the inclined angle of helical baffles under axial separation connection style is 10°. Compared with the corresponding axial separation scheme without folding, the leakage can be significantly decreased, the heat transfer coefficients h and the comprehensive indices h ⋅Δ p −1/3 of FH15(10)°, FH20(10)° and FH25(10)° are increased by 6.5%/2.3%, 12.2%/7.3% and 20.4%/13.0%, and the averaged tube wall temperatures T w are decreased by 12.5 K, 25.4 K and 54.7 K, respectively. This research can provide feasibility for the extended application of helical baffles in electric heaters or heat exchangers. [ABSTRACT FROM AUTHOR]

• A novel square-tube-layout coupling-vortex scheme is proposed in TETHX. • More uniform temperature fields are observed in coupling-vortex schemes. • Nu, f reduce but Nu∙f −1/3 rises with rising S , but all increase with rising A / B. • The smaller S and the closer A / B is to 1, the more obvious advantage of TETHX-CV. • Nu / f / Nu ∙ f −1/3 of C12.3S50 are 12.8%/15.2%/7.6% higher than those of P12.3S50. The twisted elliptical tube heat exchangers with the novel coupling-vortex chessboard tube layout (TETHXs-CV) were simulated with the RNG k - ε model in software Fluent 15.0. The shell-side flow pattern at the tangential points between adjacent tubes in traditional TETHX is countercurrent while that in TETHX-CV is concurrent, which helps enhance heat transfer capability without any extra manufacturing cost. The impacts of the ratio A / B of the major axis to the minor axis and the twist pitch S were also investigated. The results indicate that with the increase of A / B and the decrease of S , the secondary flow behaves more intensely, the Nusselt number Nu and the friction factor f increase, while with the increase of A / B and S , the comprehensive evaluation index Nu∙f −1/3 increases. The Nu, f and Nu ∙ f −1/3 of all twisted tube schemes are higher than those of the smooth round tube schemes R10.8. Compared with corresponding parallel-vortex schemes, the coupling-vortex schemes possess more uniform temperature fields, and the average Nu, f and Nu ∙ f −1/3 of the coupling-vortex schemes C12.3S50/C12.3S100 increase by 12.8%/9.9%, 15.2%/10.5% and 7.6%/6.3%, respectively; the smaller S or the closer A / B is to 1, the more advantageous the TETHX-CV. [ABSTRACT FROM AUTHOR]

• Coupling-vortex triangular tube layout is proposed without extra manufacturing cost. • Concurrent flow, larger A / B and smaller S cause stronger secondary flow. • The Nu and Nu · f −1/3 of C14.2S50 is 132.8% and 47.1% higher than R10.8. • Smaller S and smaller A / B lead to greater advantage of TETHXs-AVRCV. • The averaged Nu / Nu · f −1/3 of C12.3S50 are 7.8%/4.9% higher than those of P12.3S50. An innovative alternating V-rows triangular tube layout was devised for the twisted elliptical tube heat exchangers to enhance heat transfer between adjacent tubes. Under the same tube circumference, altogether eleven twisted elliptical tube heat exchangers with two tube layouts, diverse aspect ratios of twisted elliptical tubes and different twisted pitches were constructed and simulated, including five coupling-vortex schemes, five parallel-vortex schemes and a smooth round tube scheme R10.8. Water with constant physical properties was adopted as the working fluid. Shell-side Reynolds number varies from 2000 to 10,000. The results show that the concurrent flow and the irregular channel in coupling-vortex schemes, and the larger aspect ratio and the smaller twisted pitch of twisted elliptical tubes help intensify turbulence and secondary flow. Compared with R10.8, the averaged Nu and Nu · f −1/3 of coupling-vortex scheme C14.2S50 are improved by 132.8% and 47.1%, respectively. Under shorter twisted pitch, smaller aspect ratio and lower Reynolds number, coupling-vortex schemes can acquire superior heat transfer capability than corresponding parallel-vortex schemes but will not increase manufacturing cost. Compared with P12.3S50, the maximum and averaged Nu of C12.3S50 are increased by 9.5% and 7.8% while the maximum and averaged Nu · f −1/3 are increased by 6.4% and 4.9%. [ABSTRACT FROM AUTHOR]