Counter-flow Heat Exchanger Research Articles

Experimental and Numerical studies on the Heat transfer characteristics of Small Shell and Tube Heat Exchanger

This work presents the experimental and numerical investigations on the effectiveness of a small shell and tube heat exchanger for different coolant combinations. A numerical investigation is done using commercial computational tool, Fluent, to analyze heat transfer characteristics for the selected coolant combinations. The coolant combinations used consists of ethylene glycol (EG) and deionized water (DW) with varied percentages of the former. Accordingly, combination of ethylene glycol (EG) and deionized water viz, 60EG:40DW, 30EG:70DW and 20EG:80DW were used. The base fluids of EG and DW were used for the comparison of heat transfer characteristics, respectively with the selected coolant combination. For the numerical investigations, the CAD model of counter flow heat exchanger with 0 degree inclined baffles; with cold liquid flowing through the shell at the rate of 0.3 kg/s while hot liquid flows through the copper tubes at a flow rate of 0.3 kg/s was considered. Different combinations of shell-tube liquid are analyzed for identical initial and boundary conditions, keeping all other Fluent parameters the same. The temperature contours are generated for all the five cases. The temperature distribution for different shell-tube liquid combinations are compared for evaluating the effectiveness of heat exchange.Improved heat exchange is ensured with higher temperature difference between inlet and outlet of shell/tube, and respective combination of shell and tube fluid is selected for industrial cooling applications. Results of CFD approach are validated with the experimental results and results are acceptable. From the results it is found that 60EG:40DW combination showed improved heat transfer characteristics compared to other combinations used. Among the different coolant combinations, 60EG:40DW showed higher heat transfer as indicated by higher temperature difference across both shell and tube sections followed by other combinations when compared to pure ethylene glycol. Shell outlet and Tube outlet temperature with 60EG:40DW increased by 227% and 105% respectively when compared with pure EG, due to improved heat exchange. Heat transfer rate, heat transfer coefficient with 60EG:40DW increased when compared to pure EG.

Evolutionary Design of Compact Counterflow Heat Exchanger

Evolutionary Design of Compact Counterflow Heat Exchanger

Computational Analysis of Conventional and Helical Finned Shell and Tube Heat Exchanger Using ANSYS-CFD

The summary of the proposed work is to compare the rate of heat transfer, logarithmic mean temperature difference (LMTD) and effectiveness (ε) for a conventional shell and tube heat exchanger with and without helical finned surfaces on tube side of heat exchanger. It is shown that the inlet velocity of cold fluid at the tube side varies, while the inlet velocity of hot fluid at the shell side remains constant. The percentage variations of heat transfer rates with theoretical and simulation methods are compared. The geometry is modelled in ANSYS design modeler and analysis have been carried out in ANSYS-CFD. The inlet velocity of shell side hot fluid is varied in both types of heat exchangers. The proposed work is tested for two configurations of counter flow heat exchanger (conventional and helical finned surfaces) under different shell side inlet velocities of fluids. Helical finned tube heat exchanger with counter flow has a higher LMTD than a conventional counter flow heat exchanger. Variation in heat transfer rates and heat transfer coefficients were observed in heat exchanger influenced by shell side hot fluid velocity. The study shows that helical finned tube surfaces have improved heat transfer rates, LMTD, effectiveness (ε) and overall heat transfer coefficients compared to the conventional heat exchangers.

Heat Flux Based Optimization of Combined Heat and Power Thermoelectric Heat Exchanger

We analyzed the potential of thermoelectrics for electricity generation in a combined heat and power (CHP) waste heat recovery system. The state-of-the-art organic Rankine cycle CHP system provides hot water and space heating while electricity is also generated with an efficiency of up to 12% at the MW scale. Thermoelectrics, in contrast, will serve smaller and distributed systems. Considering the limited heat flux from the waste heat source, we investigated a counterflow heat exchanger with an integrated thermoelectric module for maximum power, high efficiency, or low cost. Irreversible thermal resistances connected to the thermoelectric legs determine the energy conversion performance. The exit temperatures of fluids through the heat exchanger are important for the system efficiency to match the applications. Based on the analytic model for the thermoelectric integrated subsystem, the design for maximum power output with a given heat flux requires thermoelectric legs 40–70% longer than the case of fixed temperature reservoir boundary conditions. With existing thermoelectric materials, 300–400 W/m2 electrical energy can be generated at a material cost of $3–4 per watt. The prospects of improvements in thermoelectric materials were also studied. While the combined system efficiency is nearly 100%, the balance between the hot and cold flow rates needs to be adjusted for the heat recovery applications.

An optimisation study on a real-world transcritical CO2 heat pump system with a flash gas bypass

The configuration of a recently installed CO2 heat pump system with a cooling capacity of 33.5 kW and heating capacity of 35.8 kW used on a dairy farm is presented. This system has been studied in order to determine the optimal performance when used for combined water heating and refrigeration. Furthermore, the effect of the interstage pressure on the CO2 heat pump system has been investigated. This required the development of a thermodynamic cycle based on the configuration of the actual constructed system and data acquired from monitoring. A compressor model with a compression ratio over three was developed and validated against experimental data with an acceptable agreement. The numerical model of the counter-flow heat exchanger between water and sCO2 has been developed assuming that the water outlet temperature is variable (just over the set water temperature), which is more consistent with actual practice. In the desktop study, the optimal COPs for heating, cooling and combined performance have been simulated considering the interstage pressure, discharge pressure and gas cooler outlet temperature. It was found that the optimal combined COPs generally decrease from 3.84 to 3.68 with the interstage pressure from 29.65 bar to 53.60 bar. Through optimisation of discharge pressure, it was found that the optimal combined COP of 4.27 occurs at the same pressure where the optimal heating COP of 2.63 is achieved (at a discharge pressure of 94 bar), while the optimal cooling COP of 1.66 occurs when the discharge pressure is around 88 bar. A further optimisation study was conducted based on a fixed cooling capacity, and it was found that the optimal cooling COP of 1.78 can be achieved when the discharge pressure is 80 bar, while the optimal heating and combined COPs are still achieved at the same discharge pressure of 93 bar (with the heating COP of 2.59 and combined COP of 4.18). Based on the simulation results, it can be summarised that the overall system thermal performance designed for heating and refrigeration is dominated by the heating performance.

Second law performance of a novel four-layer microchannel heat exchanger operating with nanofluid through a two-phase simulation

A 3D two-phase analysis is conducted to evaluate the second law features of a counter-flow four-layer microchannel heat exchanger (MCHE) operating with an Al2O3–water nanofluid. The coolant is considered water, while the hot fluid is the nanofluid. Three MCHEs with varied number of channels are examined. This investigation highlights the minimization of the exergy destruction to figure out the conditions in which the amount of degraded energy is lowest. Suspending more nanoparticles diminishes the thermal irreversibility of the nanofluid up to 61.6%, whereas the cold water experiences around 11.7% higher thermal irreversibility. At the small flow rates, increasing the volume fraction causes smaller exergy destruction, while at great flow rates, the trend becomes slightly ascending. The greater flow rate results in the greater thermal and frictional entropy generations. The second law efficiency firstly escalates via increasing the volume fraction but then remains almost constant or even descends somewhat.

An experimentally validated evaporative phase change heat transfer model for low mass flux applications using R134a in plate heat exchangers

Plate frame heat exchangers are common in liquid-coupled vapor compression systems due to their compactness and ease of maintenance. In some of these systems, the refrigerant within the evaporator can enter the heat exchanger as a subcooled liquid. As a result, the refrigerant passes through three different phases: single-phase liquid, two-phase fluid, and a superheated vapor. Unfortunately, there have been limited prior studies that have identified the best method to predict performance under these circumstances. Furthermore, previous investigations have not evaluated refrigerant evaporation in large industrial sized plate frame heat exchangers under conditions experienced in the present study experimental test facility, especially at mass fluxes below 7 kg m −2 s −1 . In the present investigation, a model was developed to predict the performance of plate and frame heat exchangers when the fluid enters as a subcooled liquid and exits as a superheated vapor. The model used full-sized plate heat exchanger geometry that was discretized into 20 sections to accurately capture local heat transfer and pressure drop effects. The model was validated using an R134a counter flow heat exchanger used in a turbo-compression cooling system test facility that had subcooled liquid entering the evaporator at low mass fluxes. A variety of empirical correlations were evaluated to determine what combination yielded the best predictive capability over the following range of conditions: 5.8 < G < 6.8 kg m −2 s −1 , 2 < q ″ < 2.8 kW m −2 , and 10 °C < T sat < 15 °C. When the correlations used a fixed control volume length in the heat flux calculation, the most accurate combination resulted in a mean absolute error of 5.5%. Future studies can use the approach described here to optimize heat exchanger size and performance.

Optimization procedure of a new type counter–flow heat exchanger

Plenty of studies exist in books and archival journals dealing with different types of heat exchangers. In the paper an analytical approach to evaluate the overall heat transfer coefficient of a new type heat exchanger is presented. Derived equations are applied to multi-objective optimization of a very large economizer of a recovery boiler, when the exchanger mass and size should be small but simultaneously heat transfer rate high.

Experimental research on the cooling performance of a new regenerative dew point indirect evaporative cooler

This paper presents an experimental investigation of a new regenerative dew point indirect evaporative cooler that uses a polymer plant fibre composite material and a pure counter-flow heat exchange structure. The wet bulb efficiency, dew point efficiency and cooling capacity of the cooler were tested and analysed under various working conditions, such as a dry area, moderate humidity area and high humidity area conditions in an enthalpy laboratory. The experimental results show that the wet bulb efficiency reaches 105.6% and the dew point efficiency reaches 76% under standard drying conditions (dry-bulb temperature of 38 °C and wet-bulb temperature of 23 °C) and when the secondary air/primary air volume ratio is equal to 1.1, the cooling capacity is 2.83 kW, the dry-bulb temperature drop can reach 15.2 °C. Simulating the experimental results of the working conditions of Urumqi and a standard dry environment shows that the application of dew point indirect evaporative cooling technology in hot and dry areas has great advantages and broad application prospects. • The cooler is made of polymer plant fiber composite material, which has high-efficiency hydrophilicity. • The cooler is a pure countercurrent heat exchange structure to improve the heat and mass transfer efficiency. • The performance of the cooler under different environmental parameters was studied experimentally. • The experimental results show that the wet bulb efficiency reaches 105.6% and the dew point efficiency reaches 76% under standard drying conditions.

Design of a compact mesh-based high-effectiveness counter-flow heat exchanger and its integration in remote cooling systems

Compact high-effectiveness Counter Flow Heat EXchangers (CFHEX) are crucial components of recuperative coolers, such as Joule-Thomson and Turbo-Brayton coolers and of remote cooling systems realised by a convective loop. This paper presents a design and analysis of a cryocooler-based remote cooling system that extends the cooling capabilities of a two-stage cryocooler. Increased heat exchange between high- and low-pressure channels is established by adding copper mesh material. A compact effective mesh-based CFHEX design covering the 4.5-290 K temperature and 1–10 bar pressure operation ranges is presented. The discretised numerical model of the CFHEX is also presented and covers a wide field of parameters, including axial conduction, variable material and fluid properties based on experimental and theoretical data and wall-mesh thermal contact conductance. In our design the latter has shown to have a significant influence on the effectiveness of the CFHEX based on the analysis of a range of inner tube materials. The sizing of a high-performance CFHEX with a predicted effectiveness of 96.5 % (number of transfer units (NTU)=27.6) and an accumulated pressure drop of 15 mbar using the model is demonstrated. The outlook for future work and experimental measurements of the parameters to complete the numerical model is presented.

Analytical study of superadiabatic small-scale combustors with a two-step chain-branching chemistry model: Lean burning below the flammability limit

A study of combustion in small-scale superadiabatic burners with a counter-flow heat exchange segment is presented for a simple two-step chain-branching chemistry model. The work is focused on the possibility of burning below the flammability limit. The investigation is carried out in the high activation energy limit on the basis of analytical solutions. The existence of multiple steady-state regimes is demonstrated and their stability properties are investigated. These analytical results are finally compared with the results of numerical simulations carried out for finite activation energies.

Experimental Comparative Analysis of Overall Heat Transfer Coefficient in Counter-Flow Heat Exchanger by using Helical Ribs

More performance or reduced the size of heat exchanger can be achieved by heat transfer enhancement technique. Tube helical ribs have been used as one of the passive heat transfer enhancement technique and are most widely used tube in a several heat transfer process. The results of the heat transfer characteristics in horizontal double pipe with helical ribs are presented. Six test section with different characteristics parameters of helical rib depth 1.0mm, 1.25mm, 1.5mm and helical rib pitch 4mm, 6mm, 8mm, are tested. Cold water and hot water are used as the working fluids in the shell side and tube side respectively. Experiments are performed under the condition of mass flow rate varying from 0.030 to 0.130kg/s for cold water and 0.040 to 0.140kg/s for hot water respectively. The inlet cold and hot water temperature are between 28- 300C and between 68-710C respectively. The results obtained from the tubes with helical ribs are compared with those without helical ribs. It is found that the helical ribs have a significant effect on the heat transfer coefficient and the heat transfer increases with the helical rib pitches and depth. Based on fitting the experimental data, on- isothermal correlations of the heat transfer coefficient and friction factor are proposed.

English

English

Catalytically stabilized combustion characteristics of methane-air mixtures in micro-scale heat-recirculating systems

In applying catalysis to real combustion systems, the modeling of chemistry and transport interactions may be the key to understanding combustion characteristics. The catalytically stabilized combustion characteristics of a methane-air mixture in a micro-scale heat-recirculating system were investigated to gain a greater understanding of the mechanisms of flame stabilization and to gain new insights into how to design the system with improved robustness and stability. The system comprised catalytically-active and catalytically-inactive channels in a counterflow heat exchange relationship. The essential factors for design considerations were determined with improved flame stability and combustion characteristics. Two primary mechanisms responsible for the loss of flame stability were discussed. The results indicated that both chemical and thermal environments are improved with the catalytically stabilized combustion method and the heat-recirculating structure. The design incorporates the best features of both catalytic combustion and thermal flame methods. The system is essentially free of mass transfer limitations. Excess enthalpy combustion can occur in an efficient and rapid manner, resulting from the injection of free radicals and heat produced by the catalytic reaction. The flow velocity, wall thermal conductivity, equivalence ratio, exterior heat losses are important factors in determining the performance of the system. Stable operation of the system is limited to a relatively wide flow regime, and the flow velocity is critical to achieving flame stability. There is an optimum wall thermal conductivity in terms of flame stability. The system with a moderate wall thermal conductivity will be most robust against the surrounding conditions. Blowout shifts homogeneous combustion downstream significantly without substantially reducing the reaction rate. Finally, engineering maps denoting flame stability were constructed, and design recommendations were made.

The Local Distribution of Temperatures and Entropy Generation Rate in an Ideal Counterflow Heat Exchanger

The process of heat exchange between two fluids of different temperatures and separated by a solid wall occurs in many engineering applications. Log mean temperature difference and effectiveness-NTU methods are widely used to assist in the design of heat exchangers. However, the two methods focus on overall analysis and cannot show the local temperature distributions. This paper obtains the mathematical solutions to the temperature profiles in an ideal counterflow heat exchanger. The aim of this research is to explain the phenomenon called the “entropy generation paradox”, which indicates a discrepancy between effectiveness and optimal entropy generation. The theoretical analysis reveals that the temperature curves are exponential functions when the heat capacity rates of the two streams are different; otherwise, the curves are linear functions. A heat exchanger is demonstrated to draw the temperature profiles under different working conditions. Local entropy generation rates are determined by the ratio of local stream temperatures in the form of a hook function. To realize a certain heat duty, there are many stream flow rate couples, and each couple results in a different entropy generation profile and obtains a corresponding total entropy generation. The helical steam generator of a high-temperature gas-cooled reactor is analyzed in this article and the principle of equipartition of entropy generation is confirmed. This principle indicates that, among the many working conditions to achieve a certain heat duty, a heat exchanger characterized by a nearly constant entropy production gives the best second law efficiency possible in order to achieve the best energy conversion.

Performance of a double-pipe heat exchanger with different met-al foam arrangements

This paper contributes to the field of improving the performance of heat exchangers using metal foam (MF) full-filled and partially/periodically-filled within the gap between the two pipes. The effect of configuration and arrangement of copper MF (15PPI and porosity of 0.95) installed on the outer surface of the inner pipe of a counter-flow double-pipe heat exchanger on the thermal and hydraulic performance was studied experimentally. The test section consisted of concentric two pipes; the inner pipe which was made of copper while the outer pipe was a Polyvinyl chlo-ride. Air was used as a working fluid in both hot and cold sides. A wide cold air flow rate range was covered from 3 to 36 m3/h which corresponds to Reynolds number (Re) range from 2811 to 31,335. The hot air flow rate was kept constant at 3m3/h. The temperature difference (ΔT) be-tween the inlet hot air and inlet cold air was adopted to be (20°C, 30°C, 40°C, and 50°C). The re-sults revealed that the higher Nusselt number (Nu) was at ΔT= 50°C and the thermal performance of the heat exchanger with the MF for all the arrangements was greater than the smooth heat exchanger. The highest and lowest friction factor was 1.033 and 0.0833 for the case 1 and 8, re-spectively, and the optimal performance evaluation criteria (PEC) was 1.62 for case 7 at Re = 2800. The Nu would be increased with a moderate increase in the friction factor by optimizing the arrangement of the MF. The two essential parameters that played an important role for in-creasing the PEC were the MF diameter and the MF arrangement along the axial length of the cold air stream.

Numerical Investigation of Hydraulic-Thermal Performance for a Double-Pipe Heat Exchanger Equipped with 45°-Helical Ribs

In this paper, the hydraulic-thermal performance of a double-pipe heat exchanger equipped with 45°-helical ribs is numerically studied. The ribbed double-pipe heat exchanger is modelled using three heights (H = 0, 2.5, 3.75, 5 mm) of 45°-helical ribs. Two numbers (4-ribs and 8-ribs) of 45°-helical ribs are attached on the outer surface of the inner pipe of the counter-flow double-pipe heat exchanger and compared with a smooth double-pipe heat exchanger. Three-Dimensional computational fluid dynamics (CFD) model for a laminar forced annular flow is performed in order to study the characteristics of pressure drop and convective heat transfer. In addition, the influence of rib geometries and hydraulic flow behaviour on the thermal performance is system-atically considered in the evaluations. The annular cold flow is investigated with the range of Reynolds numbers from 100 to 1000, with three heights of ribs at the same width (W = 2 mm) and inclined angles of (θ = 45°).The results illustrate that the average Nusselt number and pressure drop increase with an in-creasing number of ribs, the height of ribs and Reynold number, while the friction factor decreas-es with increasing Reynolds numbers. The percentage of averaged Nusselt number enhancement for three rib heights (H = 2.5, 3.75 and 5 mm) at 4-ribs is (34%, 65% and 71%), respectively, While for 8-ribs the enhancement percentage is (48%, 87% and 133%) as compared with the smooth double-pipe heat exchanger at Re = 100. The best performance evaluation criteria of (PEC) at (8-ribs, and H = 5 mm) is 2.8 at Re = 750. The attached 45-helical ribs in the annulus path can generate kind of secondary flows, which enhance the fluid mixing operation between the hot surface of the annular gap and the cold fluid in the mid of the annulus, which lead to a high-temperature distribution. Increasing the height of 45°-helical ribs lead to an increase in the sur-face area subjecting to convective heat transfer.

Performance investigation of a concentric double tube heat exchanger using twisted tape inserts and nanofluid

A horizontal double-tube counter-flow heat exchanger with and without twisted tape inserts (twist ratio of 3.2, 4, and 6) has been investigated experimentally using TiO2-H2O nanofluid. In this study, the heat transfer coefficient and friction factor of turbulent flow of TiO2-H2O nanofluid have been determined for Reynolds’ number varying from 5,000 to 25,000. Convective heat transfer coefficient for TiO2-H2O nanofluid (0.05% by vol.) is found to be higher than base fluid (4–6%). Also, an increase in the heat transfer coefficients up to 6%, 10.9%, 17.4%, and 23.4% are observed numerically for the 0.05%, 1%, 2%, and 3% volume concertation of nanoparticles of TiO2. On the other hand, when TiO2-H2O nanofluid (0.05% by vol.) is investigated using twisted tape inserts of twist ratio 3.2, the heat transfer coefficient is found to be increasing by 45%. The variation of the heat transfer coefficient for different types of nanofluids such as; TiO2-H2O, Al2O3-H2O, and CuO-H2O with 1% concentration of nanoparticles (by vol.) has also been studied at different Reynolds’ numbers. The simulated results show no significant effect of the size (30 nm and 50 nm) of nanoparticles (TiO2) on the heat transfer coefficient values of TiO2-H2O nanofluid at 0.05% volume concentration.

An Approximate Transfer Function Model for a Double-Pipe Counter-Flow Heat Exchanger

The transfer functions G(s) for different types of heat exchangers obtained from their partial differential equations usually contain some irrational components which reflect quite well their spatio-temporal dynamic properties. However, such a relatively complex mathematical representation is often not suitable for various practical applications, and some kind of approximation of the original model would be more preferable. In this paper we discuss approximate rational transfer functions G^(s) for a typical thick-walled double-pipe heat exchanger operating in the counter-flow mode. Using the semi-analytical method of lines, we transform the original partial differential equations into a set of ordinary differential equations representing N spatial sections of the exchanger, where each nth section can be described by a simple rational transfer function matrix Gn(s), n=1,2,…,N. Their proper interconnection results in the overall approximation model expressed by a rational transfer function matrix G^(s) of high order. As compared to the previously analyzed approximation model for the double-pipe parallel-flow heat exchanger which took the form of a simple, cascade interconnection of the sections, here we obtain a different connection structure which requires the use of the so-called linear fractional transformation with the Redheffer star product. Based on the resulting rational transfer function matrix G^(s), the frequency and the steady-state responses of the approximate model are compared here with those obtained from the original irrational transfer function model G(s). The presented results show: (a) the advantage of the counter-flow regime over the parallel-flow one; (b) better approximation quality for the transfer function channels with dominating heat conduction effects, as compared to the channels characterized by the transport delay associated with the heat convection.

Design and Simulation of Counter Flow Heat Exchanger with Fins and Internal Ribs

A Heat Exchanger is a device to transfer heat from fluid at higher temperature to fluid at lower temperature with highest efficiency. Designed for various applications such as preheating the water before sending it into boiler for generation of steam. This work is carried out with the primary objective of increasing the thermal efficiency of the procedure heat transfer process. There are different types of heat exchangers from which shell and tube heat exchanger is considered for this present work. High temperature water passes through the copper tubes and low temperature water flows outside the copper tubes. To enhance the rate of heat, transfer the modified design is implemented. The design consists of fins outside the copper tubes to increase the surface area in contact with outside fluid. Also, ribs are provided inside the copper tubes for increasing the turbulence along with more area of contact. This paper mainly deals with design, modelling and CFD analysis of the heat exchanger. Overall heat transfer coefficient of the fins and internal ribs of heat exchanger is based on the results of effectiveness-NTU approach and LMTD approach. Modelling of various components are presented with the help of standard modelling software, solid works. Simulation of the process is done using ANSYS. The simulated results obtained in the software are compared with the published experimental results which are very close to our modified design heat exchanger and it is found that the results are very close to experimental results.