Heat Transfer Enhancement In Tube Research Articles

An experimental study exploring heat transfer enhancement in tube in tube heat exchanger with pulsating flow

Rationalizing power consumption is crucial in air conditioning and refrigeration design. This study focuses on improving the coefficient of performance (COP) by applying pulsating flow. Experimental trials compared continuous counter and counter pulsating flow to continuous parallel flow, showing a 10% reduction in fluid temperature with continuous counter flow and a 20% reduction with counter pulsating flow. Sensible water heat rejection increased by 10.03% and 19.78% with continuous counter flow and counter pulsating flow, respectively. Overall heat transfer coefficient improved by 57.5% and 75% with continuous counter flow and counter pulsating flow. Effectiveness increased by 4.55% and 10.6%, while COP increased by 4.52% and 13.4% with continuous counter flow and counter pulsating flow, respectively. This research demonstrates the potential of pulsating flow techniques to enhance air conditioning and refrigeration efficiency.

A Modified Correlative Model for Condensation Heat Transfer in Horizontal Enhanced Tubes with R32 and R410A Refrigerants

An experimental study was performed that compared tube side condensation heat transfer characteristics of enhanced tubes (hydrophobic surface tubes (HYD), herringbone micro fin tube (HB), and a composite hydrophobic/herringbone (micro fin) tube (HYD/HB)) to the performance of a smooth tube (ST). The condensation heat transfer coefficient (HTC) was calculated from data that were recorded for smooth and enhanced tubes that had an outer diameter (OD) of 12.7 mm. Data were collected (as a function of mass flow rate) using a couple of refrigerants (R410A and R32), for saturated temperatures of 35 °C and 45 °C, with vapor qualities that ranged from 0.8 to 0.2. Several previously reported smooth tube HTC models were used to calculate values that could be compared to experimentally obtained HTC values. The correlation model that demonstrated the best accuracy (for the conditions considered) was then modified for use with the enhanced tubes from this study. Results from the modified correlation show differences with experimental values that ranged from −10% to +17%; the new modified correlation demonstrates high prediction accuracy. An accurate correlation allows the evaluation of enhanced heat transfer tubes for use in high-efficiency heat exchanger systems. The development of this new model is significant in the study of enhanced heat transfer.

Heat transfer enhancement in inserted extruded aluminum finned tubes

Heat transfer enhancement in tubes is widely applied in various industrial processes. The traditional method of heat transfer enhancement is to insert radial fins to expand the internal surface area and destroy the flow. However, there is a flue gas corridor in the central area of the tube, and the cost of expanding the inner fin is high. A centrally symmetrical bent extruded aluminum fin was developed to enhance the heat transfer in the tube. An experimental platform was constructed to test the performance of inner finned tubes and explore the process of combining extruded aluminum inner fin with steel tube. The bent extruded aluminum fins with two side fins extending to the central axis possesses the best comprehensive heat transfer performance. The heat transfer correlation of the optimal fin structure is Nu = 0.864Re0.4275Pry0.3818(Pry/Prw)?0.1004. With low cost and strong heat transfer performance, the bent extruded aluminum inner fin tube will gain wide applications in the engineering field.

Numerical analysis on heat transfer enhancement of sCO2 in the tube with twisted tape

Supercritical carbon dioxide (sCO2) has the advantages of easy to reach critical conditions, inactive chemical properties and good safety, which can be used as reactor coolant. However, the thermophysical properties of sCO2 change drastically near the pseudo-critical point. The resulting buoyancy and flow acceleration effects may cause local heat transfer deterioration in the coolant channel. Twisted tape is a good structure for heat transfer enhancement in tube. It can enhance turbulence, generate secondary flow, and disturb fluid flow boundary layer and thermal boundary layer, which can effectively mitigate the heat transfer deterioration of sCO2. The numerical analysis of sCO2 heat transfer deterioration and the heat transfer enhancement of twisted tape is presented in the article using ANSYS Fluent. The default turbulent Prandtl number model was modified to improve the calculation of turbulence generation and diffusion. The heat transfer enhancement characteristic of sCO2 with twisted tape under different working conditions was considered, and the effects of geometric characteristics were also discussed. The results indicate that when the flow rate is low and the heat flux is high, adding twisted tape can effectively suppress the heat transfer deterioration by reducing the viscous sublayer thickness and enhancing the turbulence intensity near the wall. The smaller the twist ratio, the more obvious the enhancement of heat transfer.

Research on prediction model of forming mechanical behavior of the dimpled tube based on the response surface methodology

A new multi-pitch extrusion machine for producing the enhanced heat transfer tube with dimples is proposed for the first time. The goal of the present study is to investigate the forming mechanism and establish a mechanical behavior prediction model for dimpled tubes. The response surface methodology with central composite design was adopted to identify the relationships between independent variables (extrusion depth, pitch, and teeth radius) and responses (stress, plastic strain, and excursion force). The validity of the finite element model and prediction model was verified experimentally and by the analysis of variance technique. The results show that dimples and protrusions are caused by the tube wall bending inward and outward, respectively. The stress, plastic strain, and excursion force increased with increasing extrusion depth, and decreased with increasing extrusion pitch. The changes in teeth radius had no influence on the excursion force.

Effects of trapezoidal and twisted trapezoidal tapes on turbulent heat transfer in tubes

Trapezoidal and twisted trapezoidal tape elements which are used for heat transfer enhancement in tubes are experimentally investigated. The effects of the tapes on heat transfer, pressure drop, and overall performance factor are analyzed. Three different thread heights are used in the experiments, which are obtained by dividing the tape width by 4, 6, and 10. Pitch length is kept constant for the trapezoidal tapes, while the constant twisting ratio is used for the twisted trapezoidal ones. Tapes were held in the tube with no wall contact by stretching mechanically. Experiments were conducted under constant heat flux boundary condition and for Reynolds number interval of 5000–25,000, targeting turbulent flow at which overall performance factor values reported in literature are relatively small comparing to laminar flow. The experimental results show that trapezoidal tape elements enhance heat transfer and increase pressure drop with changing rates depending on thread heights. Overall performance factor in the tested interval is detected over unity for two twisted trapezoidal inserts, making them significant alternatives comparing to the literature. The best thermal performance of 1.164 is obtained with the twisted trapezoidal tape element with the maximum thread height, while Nu is 90.52 and friction factor is 0.12. A maximum Nu of 118.46 is achieved with the proposed twisted trapezoidal tape, which is 1.9 times higher than the plain tube case and 1.57 times higher than the conventional twisted tape case. Since the twisted trapezoidal tapes perform better than the plain trapezoidal ones, it is concluded that the combination of the twisted and trapezoidal forms may be favorable in terms of heat transfer enhancement and overall performance factor regarding combination of swirl flow effect and periodical flow disturbance effect.

Experimental comparison on condensation and evaporation outside double-side enhanced horizontal tubes

Experimental investigations of the condensation and evaporation of R410A on the outside of two double-side enhanced heat transfer tubes were conducted, and the tube performance was compared with that of a smooth copper tube. The research object consisted of a smooth tube with a length of 2000 mm and an outer diameter of 25.4 mm and two double-side enhanced tubes of the same size. The evaporation experiments were run under the following conditions: the evaporation saturation temperature was 7°C, water inlet temperature was 8°C–18°C, and water flow rate was 0.6–1.6 m3/h. The condensation experiments were run under the following conditions: the saturation temperature was 35°C, water inlet temperature was 21°C–30°C, and water flow rate was 0.4–1.6 m3/h. During the data processing, the heat transfer coefficient of the water side was obtained using the Gnielinski-Wilson method, and that of the refrigerant side was obtained using the thermal resistance separation method. The effect of the surface structure on the heat transfer performance was analyzed, and a strengthening measure was proposed. The experimental results indicate that the evaporation heat transfer coefficients outside the smooth tube increased with the raise of the inlet water temperature, while the double-side enhanced tubes decreased; both the condensation heat transfer coefficients outside the smooth tube and the enhanced tubes increased with the increase of the water flow rate. Due to the different type of enhanced tube ribs, tube #3 surface structure is more conducive to the formation of vaporization core, which is about 30% higher than the outer heat transfer coefficient of tube #2. It is more in line with the requirements of strengthening horizontal single tube external pool boiling heat transfer. In the condensation experiment, the condensation heat transfer coefficients outside the smooth tube increased with the raise of the inlet water temperature, while the double enhanced tubes decreased; both the condensation heat transfer coefficients outside the smooth tube and the enhanced tubes increased with the increase of the water flow rate. With the analysis of the rib structure, the outer surface structure of tube #2 has stronger drainage capacity, and the external average condensation heat transfer coefficient is about 7% higher than that of tube #3, which is more in line with the requirements of condensing heat transfer outside the single tube.

Evaporation flow patterns and heat transfer in copper and stainless steel three-dimensional dimpled tubes

An experimental study evaluating tube side evaporation (using R410A refrigerant) in smooth and enhanced tubes was performed. Enhanced heat transfer tubes made of copper and stainless steel were studied visually and experimentally; with a nominal outer diameter of 12.7 mm and inner diameter of 11.5 mm; tube length of 1.85 m; with an enhanced surface that was composed of dimples and petal arrays. Experiments were carried out at a saturated temperature of 6°C; for a vapor quality range from 0.1 to 0.9; with mass velocities fixed at 100, 150 and 200 kg m−2 s−1. Flow patterns observed in this study included stratified flow, stratified-wave flow, annular flow and dry flow. These patterns, in addition to vapor quality and mass flux all influence the flow patterns/heat transfer, which is discussed in detail. The enhanced heat transfer structure produces the most violent disturbance and the lowest transition vapor quality between the stratified-wave flow and annular flow; additionally, the heat transfer enhancement ratio was 1.27–1.96 while the pressure drop during boiling was 11–36% higher when compared to the smooth tube. Influence of the thermal conductivity of the enhanced tubes on the flow pattern/ evaporation characteristics is also discussed; copper enhanced tube produced 17-53% larger heat transfer coefficient than stainless steel enhanced tube. The flow pattern maps suitable for enhanced tubes were modified and new heat transfer correlations were established.

Two-phase heat transfer correlations in three-dimensional hierarchical tube

Development of novel enhanced surface tubes is an important way to advance two-phase flow heat transfer. To make correlations of two-phase flow heat transfer in enhanced tubes with acceptable prediction has been a challenge for many years. The paper presents a method: two-phase heat transfer correlations in enhanced tubes are directly developed through existing two-phase heat transfer smooth-tube models by introducing several modifying factors and dimensionless numbers. An enhanced heat transfer tube containing staggered arrays of three-dimensional dimples and secondary petal-shaped protrusions (EHT) is one of the most complicated internal surface-tube configurations on the market. Visualizations of flow patterns and measurements of heat transfer coefficients of two-phase flow using refrigerant R410A in smooth and EHT tube are conducted. Detailed visualization experiments during two-phase flows show good agreements between flow regimes and flow-pattern maps in smooth and EHT tube. The equation of continuity is adopted to provide understanding of the mechanism of the agreements. Both theoretical analysis and experimental studies support the method. As a result of using it, new two-phase heat transfer correlations are developed in enhanced tubes as the EHT tube is an example. Flow boiling correlation can predict data points within +10% to -20% and flow condensation correlation can predict data points within ±10% in the EHT tube.

Numerical Study of Thermal-Hydraulic Performance Improvement of Enhanced Surface Tubes

A computational fluid dynamics study was carried out to compare the thermal-hydraulic performance of two enhanced heat transfer tubes (Vipertex models 1EHT-1 and 1EHT-2) to the performance of a smooth surface tube while using R410A as the working fluid. All tubes had an equivalent internal diameter of 8.32 mm and were made of stainless steel. Analysis was performed using an SST (shear stress transport), k-omega numerical model, with the Reynolds number ranging from 15500 to 32500. There was an increase in the heat transfer coefficient and frictional pressure drop for increasing Reynolds number. Heat transfer coefficient relative error (RE) percentage for all tubes evaluated was below 10%, while for the pressure drop the RE was below 16%. Additionally, an analysis of the boundary layer in the case of the enhanced surface tube shows a fluid flow disruption and it reveals increased fluid mixing with enhanced heat transfer. Additionally, it was discovered that dimple height has more of an effect on the thermal hydraulic performance than dimple diameter. It is possible to conclude that the SST k-omega model can be used to simulate and predict heat transfer within an acceptable error tolerance, allowing the optimization of operational parameters.

An Experimental Investigation to Determine the Effect of Tube Material on the Tubeside Heat Transfer Performance of the Enhanced 1EHT Three Dimensional Heat Transfer Tube

Condensation heat transfer characteristics were experimentally investigated over a wide range of operating conditions in order to determine the heat transfer performance inside horizontal, smooth and enhanced heat transfer tubes; using R410A in tubes produced of copper and stainless steel. Experimental data was verified and results were compared to the performance measured in a smooth tube. Results indicate that the condensation heat transfer coefficient (HTC) enhancement ratio is in the range from 1.15 to 2.05 for the 1EHT tube and for the HX tube it ranged from 1.18 to 1.69. Smooth tube heat transfer performance was slightly affected by the thermal conductivity of the tube; however, larger enhancements are found in the enhanced tubes.Heat transfer coefficients increase with an increase of mass velocities. When the mass flux increases, the liquid flow becomes more turbulent and the liquid film becomes thinner; this reduces the thermal resistance and enhances the heat transfer. Heat transfer performance for low mass velocities rise slowly, showing only a small difference in magnitude. Performance increase is larger at high mass flux rates than that those found at low mass fluxes. The influence of thermal conductivity on the condensation heat transfer of the enhanced horizontal tubes was discussed. Better heat transfer performance occurs in tubes produced of a higher thermal conductivity material (copper) or in tubes with a smaller diameter.

Entropy-Based Optimization for Heat Transfer Enhancement in Tubes With Helical Fins

Abstract In this work, a tube with internal helical fins is analyzed and optimized from an entropy generation point of view. Helical fins, in addition to providing heat transfer enhancements, have the potential to level the temperature of the tube under nonuniform circumferential heating. In this work, the geometric parameters of internal helical fins are optimized under two different entropy-based formulations. Specifically, the optimal design solution obtained through the minimization of total entropy is compared with the solutions from the multiobjective optimization of the thermal and viscous entropy contributions when considered as two separate objectives. The latter quantities being associated with heat transfer and pressure drops, it is shown that, from a design optimization point of view, it is important to separate both entropies which are conflicting objectives.

Experimental study on enhanced heat transfer tubes in falling film evaporation

The vertical tube falling film evaporation experimental platform was built, and the falling film evaporation heat transfer experiment was carried out. Comparative study was carried out between two types of enhanced heat transfer tubes, which are converging-diverging tube and the transversally corrugated tube, to explore their heat transfer characteristics of falling film evaporation. The results show that the falling film evaporative heat transfer coefficient of the two tubes increases with the increase of the unit peripheral flow rate, the heat transfer temperature difference, heat flux and the Reynolds number of the flash steam. The falling film evaporation heat transfer coefficient of the converging-diverging tube is about 1.09 times that of the transversally corrugated tube. According to the experimental data, the falling film evaporation heat transfer correlations of these two types of tubes were obtained. These results provide a useful reference for the using of transversally corrugated tube and converging-diverging tube in vertical falling film evaporator.

Numerical simulation on fluid flow and heat transfer characteristics in inward sinusoidal rib enhanced tube heat exchangers for waste heat recovery: Comparisons and parametric studies

Waste heat recovery is of great importance in industrial applications, in which heat transfer enhancement technologies play important roles. Aiming to employing heat transfer merits from longitudinal vortexes for potential waste heat recovery applications, inward sinusoidal rib tubes (SRTs) were employed as heat transfer enhancement devices, and computational fluid dynamics on tubular turbulent heat transfer and flow characteristics of the SRTs were conducted. Comparisons of thermal-hydraulic performances for a SRT with some typical enhanced heat transfer tubes and parametric studies of the SRTs involving flow structures and heat transfer mechanisms were performed. The main findings obtained from this research showed that at the similar conditions the SRT yielded higher Nusselt number (Nu), friction factor (f) and overall thermal performance evaluation criterion (PEC) than the other typical enhanced tubes. It was obtained that thermal convection in the SRTs was enhanced by the combined flow structures from transverse vortexes and longitudinal swirl flows. The results in the case studies proved that increasing rib height ratio, and decreasing pitch length ratio and rib width ratio facilitated enhancement of both Nu and f. In terms of the rib number, the SRT with the rib number of 3 gave the largest Nu, while the SRTs at the rib numbers of 2 and 3 had comparable f and they had larger flow resistance than others. In addition, the highest PEC of about 1.73 was achieved by the SRT with a pitch length ratio of 0.804, a rib height ratio of 0.065, a rib width ratio of 0.217 and a rib number of 3 at a Reynolds number of 5000.

An Experimental Study of In-Tube Condensation and Evaporation Using Enhanced Heat Transfer (EHT) Tubes

A study was carried out to determine in-tube evaporation and condensation performance of enhanced heat transfer tubes (EHT) using R410A, with the results being compared to a plain tube. The test tubes considered in the evaluation include: plain, herringbone (HB) and spiral (HX) microgrooves, herringbone dimple (HB/D), and hydrophobic herringbone (HB/HY). Experiments to evaluate the condensation were conducted at a saturation of 318 K, and at 279 K for evaporation. Mass flux (G) ranged between 40 to 230 kg m−2s−1. Condensed vapor mass decreased from 0.8 to 0.2; and the mass of vaporized vapor increases from 0.2 to 0.8; heat flux increased with G. Inlet and outlet two-phase flow patterns at 200 kg m−2s−1 were recorded and analyzed. Enhanced tube heat transfer condensation performance (compared to a plain tube) increased in the range from 40% to 73%. The largest heat transfer increase is produced by the herringbone–dimple tube (HB/D). In addition to providing drainage, the herringbone groove also helps to lift the accumulated condensate to wet the surrounding wall. Evaporation thermal performance of the enhanced tubes are from 4% to 46% larger than that of smooth tube with the best performance being in the hydrophobic herringbone tube (HB/HY). This enhancement can be attributed to an increase in the number of nucleation sites and a larger heat transfer surface area. Evaporation and condensation correlations for heat transfer in smooth tubes is discussed and compared.

Thermal performance of a boiling and condensation enhanced heat transfer tube—stepped lattice finned tube

It is a challenging job to develop a finned tube that possesses highly efficient condensation as well as boiling heat transfer enhancement. A stepped lattice finned tube (SLFT) with two layers of staggered fins produced on the side wall of spiral grooves of tube surface is designed and fabricated to achieve the goal. The overall heat transfer performance, shell side and tube side heat transfer performance of the SLFT in condensation and boiling conditions are experimentally investigated at the flow velocity ranging from 1 m/s to 3 m/s. The overall condensation and boiling heat transfer coefficients of the SLFT are up to 8.1 times and 5.1 times that of the smooth tube, respectively. Compared with other reported enhanced tubes, the SLFT has better condensation and boiling heat transfer performance. The SLFT with moderate pitch, small step height and small width possesses better condensation heat transfer enhancement whereas the SLFT with large pitch, large step height and moderate width possesses better boiling heat transfer enhancement. The SLFT with superior heat transfer enhancement under both condensation and boiling conditions provides a promising solution to highly efficient and compact heat exchanger integrated refrigerating and heating.

Evaporation Heat Transfer of R134a on outside of Smooth and Enhanced Tubes

Increasing attention has been paid to the flow and heat transfer in annular channels since their intensive applications in engineering. In this study, evaporation of the outer surface of two enhanced heat transfer tubes (2EHT) was compared to the performance of a smooth surface copper tube through experimental research. The refrigerant flowed in an annular space between the outer tube and the test tube. The outside and inside diameters of tubes used in this study were 19.05 and 17.10 mm, respectively. Tests were performed using refrigerant R134a as the working fluid, over the saturation temperature of 6 °C for evaporation, the quality range of 0.1–0.8 and the mass fluxes is from 30 to 58 kg/m2s. It is observed that through different surface structures, the heat transfer area increased, while both the nucleate boiling and convective boiling were improved, leading to the heat transfer coefficient of 2EHT being significantly enhanced. According to the evaluation, the evaporation enhanced tube had better enhancement effect. Finally, correlation analysis was conducted based on experimental data. The results from correlation analysis can be implemented to predict the experimental heat transfer coefficients within a deviation of ±10%.

Experimental Comparison of the Evaporation and Condensation Heat Transfer Coefficients on the Outside of Enhanced Surface Tubes with Different Outer Diameters

An experimental investigation was performed, and results are presented here for an evaporation and condensation heat transfer study that took place using enhanced heat transfer (1EHT) tubes of different outside diameters. Results were compared to the performance of a smooth surface copper tube. The equivalent outer diameter of the horizontal copper tubes considered were 12.7 mm (0.5 in) and 19.05 mm (0.75 in). The test apparatus included a straight horizontal test section with an active length heated by water. Constant heat flux was maintained and refrigerant quality varied.Experimental runs were performed using R410A and R134a as the working ?uids. Experimental condensation results are presented for a fixed inlet/outlet vapor quality and fixed saturation temperature, while mass flux varied; producing a heat transfer coef?cient increase (compared to smooth tube) in the range 54 to 64 %, with a frictional pressure drop increase for the 1EHT tubes in the range of 8 to 31 %. Unexpected evaporation results were found at higher mass flows; producing a decrease (compared to smooth tubes) of the heat transfer coef?cient for 1EHT tubes in the range of 100 – 220 %; with an increase of frictional pressure drop values for the 1EHT tubes in the range 5 to 17 %.Enhanced heat transfer tubes are important options to be considered in the design of high efficiency systems. A wide variety of industrial processes involve the transfer of heat energy during phase change and many of those processes employ old technology. These processes are ideal candidates for a redesign that could achieve improved process performance. Vipertex 1EHT enhanced tubes recover more energy and provide an opportunity to advance the design of many heat transfer products.

Experimental investigation on the convective heat transfer enhancement in tubes with twisted-tape inserts

Passive heat transfer enhancement techniques are one of the most interesting approaches to increase the thermal efficiency of heat transfer apparatuses since they do not require external power. Between them, twisted-tape inserts are a very economical technique that can be easily installed in an already working plant in order to increase its thermal performance. The aim of the present work is to experimentally investigate the heat transfer augmentation mechanism induced by the insertion of twisted tapes. The heat transfer performance of the twisted-tape inserts is compared with that of coiled pipes, whose effect on heat transfer is based on the onset of swirl components into the flow as it happens for twisted tapes devices. The two heat transfer enhancement mechanisms have been deeply investigated by using different fluids in order to obtain a robust comparison both for the laminar and the turbulent flow regime.

Condensation and evaporation characteristics of flows inside vipertex 1eht and 4eht small diameter enhanced heat transfer tubes

Results are presented here from an experimental investigation on tubeside condensation and evaporation heat transfer that took place in three Vipertex stainless steel enhanced heat transfer tubes (1EHT-2, 2EHT-2 and 4EHT). Equivalent outer diameter of the tube was 9.52 mm (0.375 in) and the inner diameter was 8.32 mm (0.3276 in). The test apparatus included a horizontal, straight test section with an active length heated by water circulated in a surrounding annulus. Constant heat flux was maintained and refrigerant quality varied. Condensation experimental runs were performed using R410A as the working fluid; over the inlet quality range of 0.2– 0.8; for mass flux values that ranged from 150 to 460 kg/(m2 s). In a comparison of condensation heat transfer performance, the enhanced 1EHT-1 tube has best heat performance followed by the enhanced 4EHT tube and finally the 1EHT-2 tube. The highest pressure drop increase was seen in the 1EHT-1 tube followed by the 4EHT tube and the 1EHT-2 tube. The evaporation experiments were performed using R410A at a constant saturation temperature of 279 K; for a mass flux that ranged from 160 to 390 kg/(m2 s). Inlet and outlet vapor qualities were fixed at 0.2 and 0.8, respectively. As the mass velocity increased, the heat transfer coefficient and pressure drop penalty increase accordingly. Experimental results show a slightly larger pressure drop in the 1EHT-1 tube. The pressure drop in the three EHT tubes could be attributed to the dimples and protrusions in the surface structure, which produce an increased density of nucleation sites. In addition, it was found that the evaporation pressure drop increases with the increasing depth of the dimples. Condensation and evaporation performance is mainly due to the increase in the heat transfer surface area and the increase of interfacial turbulence; this produces flow separation, secondary flows and a higher heat flux from the wall to the working fluid. Enhanced heat transfer tubes are important options to be considered in the design of high efficiency systems. A wide variety of industrial processes involve the transfer of heat energy during phase change and many of those processes employ old technology. These processes are ideal candidates for a redesign that could achieve improved process performance. Vipertex enhanced tubes recover more energy and provide an opportunity to advance the design of many heat transfer products.