Double-pipe Helical Heat Exchanger Research Articles

Heat transfer enhancement in a double-pipe helical heat exchanger using spring wire insert and nanofluid

Heat transfer enhancement in a double-pipe helical heat exchanger using spring wire insert and nanofluid

Simulation of heat transfer and effectiveness in a helical heat exchanger made from thermally enhanced polymer material for use in absorption cooling

Heat exchangers in absorption chillers are usually made of copper material. However, prob-lems of corrosion are usually encountered, especially in the solution heat exchanger. In this study a numerical investigation of the heat transfer effectiveness in a double pipe helical heat exchanger made from a thermally enhanced polymer material was conducted. The material consists of a Liquid crystal polymer (LCP), (Vectra A950) as the matrix material, while carbon fibre is the filler material. The resulting composite has a carbon fibre weight fraction of 74%. The heat exchanger was modelled as a counter flow solution heat exchanger to be used in a lithium bromide – water absorption chiller of 3 kW capacity. The numerical software ANSYS fluent (version 14.5) was used for the modelling and simulation. Thermal and mechanical properties of the thermally enhanced polymer were used in the modelling and simulation. The viscous laminar model was used, while employing a second order upwind solution method. Results indicate that the heat exchanger was able to perform the required duty by reducing the strong solution temperature from 90oC at inlet to 57oC at outlet, while increasing the weak solution temperature from 40oC at inlet to 67oC at outlet. The effectiveness of the heat ex-changer was 77.4%. Results were numerically compared to a corresponding heat exchanger of same geometry and flow conditions, made of copper. It was observed that the polymer heat exchanger attained 89.2% effectiveness of the copper heat exchanger.

Heat Transfer and Entropy Generation in Concentric/Eccentric Double-Pipe Helical Heat Exchangers

Double-pipe helical heat exchangers are integral to contemporary mechanical refrigeration equipment. Modification of flow geometry has been widely adopted to enhance heat transfer performance of a heat exchanger. The objective of this study is to numerically investigate heat transfer and entropy generation in a double pipe helical heat exchanger with various cross-sections. A computational model for laminar convective heat transfer was developed and validated against the results from previously published literature. To capture entropy generation, the entropy balance equation for open system is adopted. Effect of inner pipe Dean number, inner pipe and annulus inlet mass flow rate ratio, eccentricity, and flow configuration (co-flow and counter-flow) were examined and discussed in light of computational results. To ensure fair comparison, the considered geometries have same inner pipe cross-section area, same annulus cross-section area, and same outer surface area of inner pipe. The results suggest that square cross-section offers best performance in term of heat transfer, pressure drop and entropy generation. In addition, concentric configuration is more appropriate for low flow rate application while eccentric outer configuration is more suitable for high flow rate application.

Experimental Investigation of the Effect of Curvature Ratio on Heat Transfer in Double Pipe Helical Heat Exchanger

Different parameters of double pipe helical coil were investigation experimentally. Four coils were used; three with a curvature ratio (0.037, 0.031, and 0.028) and 11mm diameter of the inner tube while the fourth with 0.033 curvature ratio and 13 mm diameter of the inner tube. The hot water flow in the inner tube whereas the cold water flows in the annulus. The inlet temperatures of hot and cold water are 50 0C and 18 0C respectively. The inner mass flow rate ranges from 0.0167 to 0.0583 kg/s. The results show the Nusselt number increase with increase curvature ratio. The Nusselt number of the coil with 0.037 curvature ratio increases by approximately 12.3 % as compare with 0.028 curvature ratio. The results also reveal that the Nusselt number of the coil with curvature ratio 0.033 increases by approximately 11.6 % as compare with 0.028 curvature ratio for 400 mm coil diameter.
 

Experimental Investigation of the Effect of Curvature Ratio on Heat Transfer in Double Pipe Helical Heat Exchanger

Different parameters of double pipe helical coil were investigation experimentally. Four coils were used; three with a curvature ratio (0.037, 0.031, and 0.028) and 11mm diameter of the inner tube while the fourth with 0.033 curvature ratio and 13 mm diameter of the inner tube. The hot water flow in the inner tube whereas the cold water flows in the annulus. The inlet temperatures of hot and cold water are 50 0C and 18 0C respectively. The inner mass flow rate ranges from 0.0167 to 0.0583 kg/s. The results show the Nusselt number increase with increase curvature ratio. The Nusselt number of the coil with 0.037 curvature ratio increases by approximately 12.3 % as compare with 0.028 curvature ratio. The results also reveal that the Nusselt number of the coil with curvature ratio 0.033 increases by approximately 11.6 % as compare with 0.028 curvature ratio for 400 mm coil diameter.
 

Experimental Investigation of the Effect of Curvature Ratio on Heat Transfer in Double Pipe Helical Heat Exchanger

Different parameters of double pipe helical coil were investigation experimentally. Four coils were used; three with a curvature ratio (0.037, 0.031, and 0.028) and 11mm diameter of the inner tube while the fourth with 0.033 curvature ratio and 13 mm diameter of the inner tube. The hot water flow in the inner tube whereas the cold water flows in the annulus. The inlet temperatures of hot and cold water are 50 0C and 18 0C respectively. The inner mass flow rate ranges from 0.0167 to 0.0583 kg/s. The results show the Nusselt number increase with increase curvature ratio. The Nusselt number of the coil with 0.037 curvature ratio increases by approximately 12.3 % as compare with 0.028 curvature ratio. The results also reveal that the Nusselt number of the coil with curvature ratio 0.033 increases by approximately 11.6 % as compare with 0.028 curvature ratio for 400 mm coil diameter.
 

Experimental studies of heat transfer of air in a double-pipe helical heat exchanger

The overall heat transfer coefficient (OHTC) of air in a double-pipe helical heat exchanger is studied and experimentally investigated. In order to increase the rate of heat transfer in the annulus section, a copper-wire fin is soldered on the outside area of the internal tube. The literature correlations’ results are presented against the experimental data by changing mass flow rate and temperature, and consequently changing the Reynolds number. Further, a new method is suggested to obtain the heat transfer coefficients of double-pipe helical heat exchangers for the internal tube and the annulus section by combining two of the previously proposed approaches and making subsequent corrections, e.g. using equivalent diameter instead of hydraulic diameter. Moreover, the influence of the hot and cold mass flow rates on the OHTC, as well as the effect of the installed fin on the heat transfer coefficient of the annulus section are investigated. Finally, obtained results show the strength of the suggested correlation and the enhancement in the OHTC due to the presence of the fin in the annulus.

Best vortex tube cascade for highest thermal separation

The current study examines different arrangements of vortex tubes (VTs) to get higher performances for cooling and heating. The effects of thermo-physical parameters such as inlet feed temperature and inlet/outlet vortex tube pressure on generated temperature gradient are investigated. To estimate the cold outlet temperatures, the available equations in the literature are verified against our experimental data. Moreover, we propose a new equation to estimate the hot outlet temperature based on the upper limit of hot temperature (ULHT) and the lower limit of cold temperature (LLCT), verified with experimental data as well. Further, several arrangements are simulated to obtain the minimum cold and the maximum hot temperatures for the outlet streams. Finally, the optimal arrangement of the three vortex tubes and a double-pipe helical heat exchanger that gives the highest performance is proposed.

Pressure drop and convective heat transfer of water and nanofluids in a double-pipe helical heat exchanger

Pressure drop and convective heat transfer characteristics of water and five alumina/water nanofluids of weight concentrations from 0.78% wt. to 7.04% wt. were experimentally investigated for both laminar flow and turbulent flow inside a double-pipe helically coiled heat exchanger. Effect of nanoparticles on the critical Reynolds number is negligible. A new correlation was developed for laminar flow in helically coiled tubes, which can predict the experimental heat transfer data very well. For turbulent flow, the Seban and McLaughlin correlation can accurately predict the thermal behavior of water and nanofluids when nanofluid properties are taken into account. For both laminar flow and turbulent flow, no anomalous heat transfer enhancement was found. The heat transfer enhancement of the nanofluids compared to water is from 0.37% to 3.43% according to the constant flow velocity basis. Figure of merit based on the constant Reynolds number can be misleading and should not be used for heat transfer enhancement comparison. Additional possible effects of nanoparticles, e.g., Brownian motion, thermophoresis and diffusiophoresis, on the convective heat transfer characteristics of the nanofluids are insignificant compared to the dominant thermophysical properties of the nanofluids. No multiphase phenomenon was found and the tested alumina nanofluids can be treated as homogeneous fluids.

Numerical analysis of the lethality and processing uniformity in a double-pipe helical heat exchanger

A numerical study of a double-pipe helical heat exchanger was performed to ascertain the residence time, temperature, and processing uniformity for food processing applications. A range of laminar flow rates were used, with both parallel flow and counterflow configurations. Both heating and cooling in the inner tube were studied. Heating/cooling uniformity was estimated by using a first-order kinetics model for sterilization. Process uniformity is important in the quest for high quality product and this report is a first study for the uniformity in double-pipe helical heat exchangers. Residence time distributions became more uniform with increased flow rates in both the inner tube and in the annulus. Temperature uniformity in the inner tube increased with increasing flow rate. However, it was shown that changing the flow rate in the annulus, for a constant flow rate in the inner tube, affected the uniformity. Similar results were found for the annulus. Heating uniformity in the inner tube tended to increase with increased flow rates. It was also shown that the cooling process was more uniform than the heating process. The findings in this study are important to the design of helical heat exchangers for the processing of liquid foods.

Thermally dependent viscosity and non-Newtonian flow in a double-pipe helical heat exchanger

A double-pipe helical heat exchanger was numerically studied to determine the effects of thermally dependent viscosity and non-Newtonian flows on heat transfer and pressure drop for laminar flow. Thermally dependent viscosities were found to have very little effect on the Nusselt number correlations for Newtonian fluids; however significant effects on the pressure drop in the heat exchanger were predicted. Changing the flow rate in the annulus can significantly affect the pressure drop in the inner tube, since the average viscosity of the fluid in the inner tube would change due to the change in the average temperature. The effects of non-Newtonian power law fluids on the heat transfer and the pressure drop were determined for laminar flow in the inner tube and in the annulus. The Nusselt number was correlated with the Péclet number for heat transfer in the inner tube. For the annulus, the Nusselt number was found to correlate best with the Péclet number and the curvature ratio. Pressure drop data were compared by using ratios of the pressure drop of the non-Newtonian fluid to a Newtonian fluid at identical mass flow rates and consistency indices.

Effect of fluid thermal properties on the heat transfer characteristics in a double-pipe helical heat exchanger

Heat transfer characteristics of a double-pipe helical heat exchanger were numerically studied to determine the effect of fluid thermal properties on the heat transfer. Two studies were performed; the first with three different Prandtl numbers (7.0, 12.8, and 70.3) and the second with thermally dependent thermal conductivities. Thermal conductivities of the fluid were based on a linear relationship with the fluid temperature. Six different fluid dependencies were modeled. Both parallel flow and counterflow configurations were used for the second study. Results from the first study showed that the inner Nusselt number was dependent on the Prandtl number, with a greater dependency at lower Dean numbers; this was attributed to changing hydrodynamic and thermal entry lengths. Nusselt number correlations based on the Prandtl number and a modified Dean number are presented for the heat transfer in the annulus. Results from the second part of the study showed that the Nusselt number correlated better using a modified Dean number. The counterflow configuration had higher heat transfer rates than the parallel flow, but the ratio of these differences was not different when comparing thermally dependent properties and thermally independent properties.

Numerical studies of a double-pipe helical heat exchanger

A double-pipe helical heat exchanger was numerically modeled for laminar fluid flow and heat transfer characteristics under different fluid flow rates and tube sizes. Two different tube diameters were used. The overall heat transfer coefficients were calculated for both parallel flow and counterflow. Validation of the simulations was conducted by comparing the Nusselt numbers in the inner tube with those found in literature; the results fell within the range found in the literature. The greatest thermal resistance was found in the annular region. The annulus Nusselt number was correlated with a modified Dean number, and showed a strong linear relationship.

Experimental studies of a double-pipe helical heat exchanger

An experimental study of a double-pipe helical heat exchanger was performed. Two heat exchanger sizes and both parallel flow and counterflow configurations were tested. Flow rates in the inner tube and in the annulus were varied and temperature data recorded. Overall heat transfer coefficients were calculated and heat transfer coefficients in the inner tube and the annulus were determined using Wilson plots. Nusselt numbers were calculated for the inner tube and the annulus. The inner Nusselt number was compared to the literature values. Though the boundary conditions were different, a reasonable comparison was found. The Nusselt number in the annulus was compared to the numerical data. The experimental data fit well with the numerical for the larger heat exchanger. But, there were some differences between the numerical and experimental data for the smaller coil; however these differences may have been due to the nature of the Wilson plots. Overall, for the most part the results confirmed the validation of previous numerical work.