Helical Finned Tubes Research Articles

Numerical study on heat transfer and flow characteristics of coaxial geothermal heat exchangers with helical finned inner tubes

In recent years, geothermal energy as well has become the first choice for building heating. In order to improve the heat transfer performance of the coaxial GHE, the flow characteristics and heat transfer characteristics of the coaxial GHE when the inner tube of the coaxial GHE is assembled with HF of different width ratio (a), thickness ratio (b) and pitch ratio (S) are numerically studied in this paper. Water is chosen as the working medium, and the Reynolds number range is 10000–60000. HF have a significant impact on heat exchanger performance by changing the fluid flow structure and improving the efficacy of heat transfer between the fluid and the wall. Numerical results show that as the Reynolds number increases, the Nusselt number (Nu) of coaxial GHE with different a, b, and S increases, while the friction factor(f) and thermal enhancement factor (TEF) decreases. The Nu and f of the coaxial GHE with HF of different a, b, and S on the inner tube are higher than those of the coaxial GHE with smooth inner tube. The change in S of HF has a greater effect on Nu and f than the changes in a and b of HF have on Nu and f. To discover the enhanced heat transfer mechanism of the coaxial GHE with HF assembled on the inner tube, flow lines, velocity, turbulence, turbulence, and temperature distribution are provided. According to the analysis, the blocking of the HF causes the fluid to appear as a spiral rotating flow structure, which improves fluid mixing and heat exchange between the fluid and the wall. Increases in both a and b of HF can improve the heat transfer strength of the fluid to the wall, while decreases in S of HF can hasten fluid mixing. Finally, two correlations are established based on the numerical simulation data to predict Nu and f as functions of design parameters in turbulent flow states.

Hydrodynamics in bubble columns with helically-finned tube Internals: Experiments and CFD-PBM simulation

Experiments and CFD-PBM simulation were performed in this study to investigate the effects of helically-finned tube (HFT) internals on the gas-liquid flow in a laboratory-scale bubble column. Hydrodynamic parameters, including gas holdup, bubble size and flow field as well as turbulence properties, were compared for an empty column and other two columns with bare-tube (BT) or HFT internals. We found that bare rods promote the stability of two-phase flow, whereas helical fins destabilize the flow. Meanwhile, the spatial distribution of gas holdup or bubble size becomes more uniform in the presence of HFT internals, whereas liquid velocity or turbulent dissipation rate evidently decreases. The fin-induced spiral movement, flow resistance and bubble accumulation are the mechanisms underlying these experimental observations. Integration of rod and helical fin may be promising for process intensification in bubble columns.

Production of Helical Finned Tubes with Various Curvatures Using Extrusion Process through Inclined Die Aperture

Production of Helical Finned Tubes with Various Curvatures Using Extrusion Process through Inclined Die Aperture

Experimental investigation on the friction characteristics of water–ethylene glycol mixture flow in internal helical finned horizontal tubes

Internal helical finned tubes are widely applied in the HVAC field because they enhance convective heat transfer. In this study, an experimental setup was built and used to investigate the single-phase flow characteristics of internal helical-rib roughness covering laminar and transition regimes. The single-phase flow characteristics of two internal helical finned tubes were also determined. The setup contained two test tubes, with nominal diameters of 22.48 and 16.662mm, number of fins of 60 and 38, helix angles of 45° and 60°, and relative roughness values of 0.022 and 0.053, respectively. The Prandtl number varied from 15 to 177, and the Reynolds number ranged from 200 to 36000. Friction factors were accurately measured under the adiabatic condition. Analysis results of the plain tube were in agreement with those obtained using the Filonenko equation. The first critical Reynolds numbers that judged the transition from laminar to transition regimes were 2160 for Tube-1 and 2070 for Tube-2. The second critical Reynolds numbers that judged the start of fully turbulent regime were 14300 for Tube-1 and 9500 for Tube-2. The entrance region influenced the friction factor of helical finned tubes. A correlation was developed to predict the friction factor in the transition regime of helical finned tubes for developing flow.

Numerical investigation of fluid flow and heat transfer characteristics in a helically-finned tube

In order to investigate the characteristics of flow and heat transfer rate in a Helically-finned tub (HFT), we used continuity, momentum and energy equations under a steady, three-dimensional and incompressible fluid flow assumptions. For the performance metrics, we considered the Darcy friction factor, Colburn j-factor, volume goodness factor and area goodness factor of the HFT. We could also evaluate the effect of geometry parameters on the results of local pressure coefficient, fluid vorticity and Nusselt number of the HFT. We carried out the CFD calculation for a range of laminar flow (Re = 100) and turbulent flow (Re = 2000 and 10000). In a laminar and turbulent flow regime, the friction factor increases with increasing the each geometric parameter. While the Colburn j-factor decreases as increasing these geometric parameters. Consequently, the thermal performance of HFT is poorer than that of single straight circular tube type because of having a small volume and area goodness factor as increasing the Reynolds numbers.

Experimental Study of Single-phase Pressure Drop Characteristics in Horizontal Internal Helical Finned Tubes

Due to the intense enhancement of the convective heat transfer, internal helical finned tubes are widely applied into the field of HVAC. An experimental setup was built for investigating single-phase flow characteristics of internal helical-rib roughness. The single-phase flow characteristics for water-ethylene glycol mixture in three internal finned tubes and a smooth tube were obtained. The nominal diameter of the two test tubes was 22.48mm and 16.662mm, the number of the fins was 60 and 38, the helix angle was 45 degrees and 60 degrees and the relative roughness was 0.022 and 0.053, respectively. The Prandtl number varied from 10.0 to 177 and the Reynolds number ranged from 185 to 40000. The friction factors were measured under the condition of thermal insulation. Experimental results showed that the smooth-tube results were compared to the Filonenko equation with satisfactory agreement. Transition for internal helical finned tubes occurred at lower Reynolds numbers than that of their smooth tube counterparts. The trend of the friction factor of the internal helical finned tube is remarkably different from that of plain tube in the transition regime. For Tube 2 and Tube 3, the flow turns into the fully turbulent flow at Re=14500, while Tube-4 at Re=9500, which is obviously different from that of the plain tube.

Experimental study of single-phase flow and heat transfer characteristics for a horizontal internal helically-finned tube

Helical fin roughness has been extensively used in the large heat exchangers. The single-phase flow and heat transfer characteristics in an internal helically-finned tube are determined experimentally in this study. The main parameters of the internal helically-finned tube is fin height to the tube inside diameter ratio (0.022), number of fins (60) and helix angle (45°). The adiabatic friction factor was collected and heat transfer experiments were conducted in a flooded evaporator with R134a boiling outside the tube. The aqueous ethylene glycol was used as the working fluid and the tests were performed with Prandtl number from 17.8 to 36.4 and Reynolds number from 8000 to 34,000. In the flow parameter range, this enhanced tube provided a 2.79-3.56 heat transfer enhancement ratio and 1.58-2.3 for friction factor and efficiency index varied from 1.77 to 1.55. This study offers the reference on the design of heat exchangers.

Experimental determination of single-phase pressure drop and heat transfer in a horizontal internal helically-finned tube

The single-phase pressure drop and heat transfer in an internal helically-finned tube and a plain tube are measured for aqueous ethylene glycol. Pressure drop data are collected under isothermal condition and heat transfer experiments are conducted in a flooded evaporator with R134a boiling outside the tube. The tests are performed with Prandtl number from 17 to 29 and Reynolds number from 5000 to 34,000. The main parameters of the internal helically-finned tube are nominal inside diameter (22.48mm), fin height to the tube inside diameter ratio (0.022), number of fins (60), helix angle (45°). It is found that plain tube’s results agree well with Filonenko and Gnielinski correlations. Friction factors of internal helically-finned tube increase with Re below 17,000, reaching a maximum at Recr=17,000 and decrease above 17,000. This indicates that the conventional criterion Re>10,000 for distinguishing turbulent flow isn’t suitable for internal helically-finned tubes. A correlation is developed to predict the experimental heat transfer coefficient within 5%. In evaluation of the thermal performance, the j-factor for internal helically-finned tube is about 3.5 times of that for the plain tube and the efficiency index varies from 1.8 to 1.55. This study offers the reference on engineering design and later study.

Numerical study on performance of molten salt phase change thermal energy storage system with enhanced tubes

Based on enthalpy method, numerical studies were performed for high temperature molten salt phase change thermal energy storage (PCTES) unit used in a dish solar thermal power generation system. Firstly, the effects of the heat transfer fluid (HTF) inlet temperature and velocity on the PCTES performance were examined. The results show that although increasing the HTF inlet velocity or temperature can enhance the melting rate of the phase change material (PCM) and improve the performance of the PCTES unit, the two parameters will restrict each other for the fixed solar collector heat output. Then three enhanced tubes were adopted to improve the PCTES performance, which are dimpled tube, cone-finned tube and helically-finned tube respectively. The effects of the enhanced tubes on the PCM melting rate, solid–liquid interface, TES capacity, TES efficiency and HTF outlet temperature were discussed. The results show that compared with the smooth tube, all of the three enhanced tubes could improve the PCM melting rate. At the same working conditions, the melting time is 437.92min for the smooth tube, 350.75min for dimpled tube which is reduced about 19.9% and 320.25min for cone-finned tube which is reduced about 26.9% and 302.75min for helically-finned tube reduced about 30.7%. As a conclusion, the thermal performance of PCTES unit can be effectively enhanced by using enhanced tube instead of smooth tube. Although, the HTF pressure drops for the enhanced tubes are also larger than that of the smooth tube, the largest pressure drop (1476.2Pa) is still very lower compared with the working pressure (MPa magnitude) of the dish solar generation system. So, the pressure drops caused by the enhanced tubes could almost be neglected.

Experimental research of CFB ash deposition on helical finned tubes

Abstract In order to evaluate the influence of ash deposition on heat transfer, experiments focusing on the influence of ash deposition on heat transfer are carried out in a 75t/h CFB(circulating fluidized bed) boiler unit. The test bundles are made of helical finned tubes in staggered or in-line arrangement. The fouling factor ɛ and the overall heat transfer coefficient ratio ψ are employed to evaluate the influence of ash deposition. The fitting correlations of ɛ and ψ under different flue gas velocities are obtained respectively. The experiment reveals that the fouling factor decreases and the overall heat transfer coefficient ratio increases with the rising flue gas velocity. Compared with the in-line arrangement, the staggered arrangement helps to lighten ash deposition. The fouling factors obtained from the present experiments are one-order smaller in the magnitude than the recommended values that are widely used in China at present. And the present results are of the same order as those gained from cold test with sands instead of fly ash. Compared with the existing research, the present results are more suitable for application in practice.

Design of solar powered adsorption heat pump with ice storage

The design and performance of a solar (and/or natural gas) powered adsorption (desiccant-vapor) heat pump for residential cooling (and heating) is described. The entire system is modeled and analyzed: adsorption heat pump itself, ice thermal storage reservoir, and solar collectors. The adsorption heat pump embodies patent pending improvements to the state-of-the-art which elevate coefficient of performance for cooling from a maximum of 1.2 reported in the literature to a conservatively predicted minimum of 1.5. The adsorption device utilizes economical, robust configurations (shell-and-tube) and components (helical annular finned tubes, multi-lumen tubes) commonly employed in heat exchangers in a manner heretofore untried, as well as other enhancements (metal wool to diffuse heat throughout the adsorbent). The vessel is all aluminum and the adsorbent-refrigerant pair is carbon-ammonia. The ice reservoir provides 24 h cooling. Two types of solar collector are determined to be satisfactory at the selected operating temperature of 170 °C: (1) compound parabolic concentrator with high concentration ratio (10+) and automatic tilt adjustment, and (2) evacuated (0.001 atm) flat panel, similar to atmospheric pressure versions employed for domestic water heating.

Research on air pressure drop in helically-finned tube heat exchangers

In order to establish a reliable procedure for estimation of air pressure drop, extensive investigation of the open literature has been conducted. The equations from mostly cited literature sources were tested against the experimental data given in the open literature and certain level of uncertainty was found. Using published experimental data, new correlations for estimation of air pressure drop in helically-finned tube bundles with in-line and staggered tube arrangement have been established. Chosen form of correlations successfully describes operating regimes for wide range of Reynolds number and geometrical parameters.

An experimental study of convective heat transfer from extruded type helical finned tubes

The hydraulic–thermal investigation were carried out for nine different staggered bundles made of bimetallic helical high finned extruded type tubes under the conditions of air flow perpendicular to horizontal staggered banks of tubes. For the flow in tubes water, or for the temperatures up to 310 °C, oil (Iterm 6Mb) was used. For the sake of determining the value of heat transfer coefficient (HTC) the use was made of the ε-NTU method. Correlations have been worked out allowing for the effect of geometric parameters and tube bundles on HTC and air pressure drop. The gas-side HTCs obtained, were higher by approximately 20% with respect to the correlations presented in the literature. These coefficients were determined at the pressure drops, close to those near in literature.