U-tube Heat Exchanger Research Articles

Research on Hydraulic Characteristics of U-Tube Heat Exchanger Loop in Ground-Coupled Heat Pump System

There were two urgent problems that were reducing hydraulic losses of loop and exploring the spatial distribution of soil heat accumulation in the ground-coupled heat pump system. In order to solve these two problems, pressure drop of water loops would be analyzed in this paper. The pressure drop and the unbalance rate of the water loop were calculated while the velocity of the U-tube was calculated. It is found that the hydraulic characteristics can be improved. When pipe diameter and length are changed, their maximum pressure drops of water loop decrease by 61%. When installation method of U-tube is chosen, different flow velocity would lead that each water loop has a different pressure drop, and the increments of pressure drop are 50%, but the imbalance rate does not change. In the same condition, the velocity of the central pipe is lower than that of boundary pipe in U-tube group. This phenomenon is very similar to spatial distribution difference of soil heat accumulation.

Thermal stress analysis on the thick tubesheet with square layout of tubes

In this paper, finite element analyses were carried out on thermal stresses at the tubesheet of floating-head or U-tube heat exchangers with square layout of tubes based on parametric finite element models of the tubesheet established with ANSYS parametric design language (APDL). It is found that for thick tubesheet with large temperature difference between two the sides, the maximum thermal stress can be comparable with the mechanical stress induced by pressures and should not be neglected in some circumstances. When hot fluid flows on the tube-side, both the shell-side and tube-side surfaces of the tubesheet are in tensile stress state while the region in the middle of the tubesheet is in compressive. Conversely, if the cold fluid flows on the tube-side, compressive thermal stresses are produced at the two tubesheet surfaces, which is helpful to prevent the tubesheet from cracking. By changing design parameters in APDL commands, parametric analysis was completed and the results show that the thermal stresses at the surface center of the tubesheet are sensitive to the thickness and the perforated region size of the tubesheet. Based on sufficient numerical results, a regressed formula for evaluating the thermal stress at the surface center of the tubesheet with squarely arranged tubes was obtained, this can facilitate the strength design of the tubesheet in floating-head or U-tube heat exchangers when considering the thermal stress.

A simulation-based analysis of variable flow pumping in ground source heat pump systems with different types of borehole heat exchangers: A case study

A simulation model of ground source heat pump systems has been used to investigate to what extent a variable flow of the heat-carrier fluid of the ground loop affects the energy efficiency of the entire system. The model contemporaneously considers the borehole heat exchangers, the heat pump, the building load, and the control strategies for the circulation pumps of the ground loop. A constant speed of the circulation pumps of the ground loop was compared with a variable flow controlled by means of a constant temperature difference across the heat pump on the ground side considering the load profile of an office building located in North Italy. The analysis was carried out for a single U-tube, double U-tube and coaxial pipe heat exchangers. The control strategies adopted to manage the flow rate of the heat-carrier fluid of the ground loop affect both the heat exchange rate of the borehole field and the heat pump’s long-term energy efficiency. The simulations show considerable differences in the system’s seasonal energy efficiency. The constant speed of the circulation pumps leads to the best results as far as the heat pump’s energy performance was concerned, but this advantage was lost because of the greater amount of electrical energy used by the circulation pumps; this, of course, affects the energy efficiency of the entire system. The optimal solution appears then to be a constant temperature difference in the heat-carrier fluid across the heat pump.

Finite-element analysis of the fluid temperature distribution in double U-tube Borehole Heat Exchangers

In the evaluation of Thermal Response Tests (TRTs) and in the design of Borehole Heat Exchanger (BHE) fields, the mean temperature of the fluid Tm is usually approximated by the arithmetic mean Tave of inlet and outlet temperatures. This approximation can yield errors in the estimation of the thermal conductivity of the ground, of the BHE thermal resistance, and of the heat pump performance. An expression for the evaluation of Tm has been proposed by Marcotte and Pasquier (Marcotte D, Pasquier P 2008, Renewable Energy, 33 2407) for single U-tube BHEs. In this paper, the difference between Tm and Tave is determined by 3D finite- element simulations for a typical double U-tube BHE in 6 unsteady working conditions. The results are validated qualitatively through an approximate analytical method, and show that the expression proposed by Marcotte and Pasquier underestimates the difference between Tm and Tave when applied to a typical double U-tube BHE. Therefore, new relations to evaluate this difference for double U-tube BHEs would be useful.

A Proposed General Method of Stress Analysis for Tubesheet of Heat Exchanger

The stress analysis method for fixed tubesheet (TS) heat exchangers (HEX) in pressure vessel codes such as ASME VIII-1, EN13445, and GB151 all assume a geometric and loading plane of symmetry at the midway between the two TSs so that only half of the unit or one TS is need to be considered. In this study, the midplane symmetry assumption is discarded. More common situations are considered such as unequal TS thickness, different edge conditions, pressure drop, and dead weight on two TSs. Based on the classical thin plate and shell theoretical solution, an analytical method of stress analysis for TS is presented. The proposed method is suitable for different types of HEX due to fewer assumptions employed in this study. Analysis shows that floating and U-tube HEX are the two special cases of the proposed method. Theoretical comparison shows that ASME method can be obtained from the special case of the simplified mechanical model of the proposed method. Typical geometries and loading are considered, and the proposed method is used to check the adequacy of design. Predictions are compared with the results obtained from axisymmetric finite element analysis (FEA) and current ASME method. Comparison results indicate that predictions given by this paper agree well with FEA while ASME results are not correct or not accurate.

Performance of ground-source heat exchangers using short residential foundation piles

Experiments on the performance of ground-source heat exchangers using short residential foundation piles were performed in Saga, Japan. U-tube, double-tube, and multitube heat exchangers were installed in the ground to depths of 20 m. Water was used as the working fluid. Temperatures were measured on pipe walls, pile walls, and in the ground. From the measured data, thermal resistances of pipes, grout, and soil were calculated. We also investigated the effect of the ground surface temperature on heat transfer rate. Total thermal resistances of the double-tube, multitube, and U-tube exchangers were 0.231 (m K)/W, 0.295 (m K)/W, and 0.356 (m K)/W, respectively. Of these total resistances, soil, grout, and pipe thermal resistances accounted for 89%, 0%, and 11%, respectively, for the double-tube heat exchanger; 50%, 27%, and 23%, respectively, for the multitube heat exchanger; and 62%, 21%, and 17%, respectively, for the U-tube heat exchanger. Heat transfer rates of the tested heat exchangers could be predicted within 8.6% by using a measured value for the surface temperature coefficient. For atmospheric temperatures around 30 °C, the effect of the surface ground temperature on heat transfer rates was approximately 10%–15%.

Experimental investigation on enhancement of heat transfer using Al2O3/water nanofluid in a u-tube with twisted tape inserts

An investigation is made experimentally with an attempt to enhance the rate of heat transfer of Al2O3 nanofluid in a U-tube heat exchanger with twisted tape insert. The heat transfer coefficients and the corresponding friction factors needed for performance analysis are determined at variable operating conditions of the heat exchangers with particle volume concentration of 0.01% and 0.03% and twist ratios ranging between 5 and 20. Experimental data are obtained at flow rates ranging from 0.0333 to 0.2667kg/s and Reynolds number ranging from 3000 to 30,000. The Nusselt number of entire pipes for 0.03% concentrations of nanofluid with twisted tape inserts is enhanced by 31.28% compared to water while the friction factor is increased by 1.23 times compared to water with twisted tape insert of H/D=5. Correlations are developed to estimate the Nusselt number and friction factor. The results of the investigation indicated significant improvement in the performance parameters of the heat exchanger with a rise in volume concentration of the nanoparticle.

Research on U-tube Heat Exchanger with Shape-stabilized Phase Change Backfill Material

Fluent software was used to simulate the heat transfer performance of the U-tube heat exchangers with backfill materials of shape-stabilized phase change materials (PCMs) and crushed stone concrete in this paper. The shape-stabilized PCMs refer to a mixture of decanoic acid and lauric acid that the mass concentration of decanoic acid is 60% with10% silica and 6% expanded graphite. It makes the shape-stabilized PCM has the coefficient of thermal conductivity of 1.528W/(m•K) and the latent heat of 109.2kJ/kg. After the simulation of the time for 12hours, the heat exchange for unit borehole depth of backfilling with shape-stabilized PCM is 1.23 times of the heat exchange for unit borehole depth of backfilling with crushed stone concrete. And the influence radius of backfill materials of shape-stabilized PCM is 0.9 times of the influence radius of backfill materials of crushed stone concrete. So under same area of buried pipes region the shape-stabilized PCM backfill can get heat exchange is 1.37 times of crushed stone concrete backfill. In addition, the heat conductivity coefficient of PCMs has great influence on heat pump coefficient.

Operation Analyse of a U-tube Ground Heat Exchangers of GSHP System in Cold Region of China

The objective of this study is to discuss summer operation performance of a large Ground Source Heat Pump System (GSHP) for a NZEB office building in cold region of Beijing, China. Borehole performance under different field distribution and its impact to the surrounding soil at different depth is monitored and analysed. It is found that double U-tube has approximately 1.3 times heat releasing amount than the single tube and soil temperature raised by up to 1.4°C after 2.5 months’ cooling operation. The operational data is of great value to understand and further optimize the operation strategy. It is believed that better system performance could be achieved with the completion of system commissioning and integration of building automation.

Heat Transfer and Friction Factor of Al<SUB>2</SUB>O<SUB>3</SUB> Nanofluid Flow in a Double Pipe U-Tube Heat Exchanger and with Longitudinal Strip Inserts: An Experimental Study

Heat Transfer and Friction Factor of Al<SUB>2</SUB>O<SUB>3</SUB> Nanofluid Flow in a Double Pipe U-Tube Heat Exchanger and with Longitudinal Strip Inserts: An Experimental Study

Numerical and experimental validation of a new hybrid model for vertical ground heat exchangers

A numerical and experimental validation of a new simplified model to simulate single U-tube ground heat exchangers is presented in this paper. The model is based on the use of the electrical analogy to model heat transfer within the borehole and thermal response factors (short and long time-step g-functions) to estimate heat flow to the surrounding ground. The substitution of ground nodes with short and long-term g-functions allows the simulation with short time steps, keeping the possibility of simulating periods of several years. The model has been validated experimentally by using the reference data sets drawn from two tests (constant heat input rate and interrupted tests) made under controlled laboratory conditions. The results have also been compared with the simulated data from a detailed Computational Fluid Dynamics model of the experimental system. A very good agreement has been observed for the modeled outlet fluid and borehole temperatures, with root mean square and relative error values smaller than 0.2°C and 0.3%, respectively.

A spectral model for transient heat flow in a double U-tube geothermal heat pump system

This paper introduces a semi-analytical model based on the spectral analysis method for the simulation of transient conductive-convective heat flow in an axisymmetric shallow geothermal system consisting of a double U-tube borehole heat exchanger embedded in a soil mass. The proposed model combines the exactness of the analytical methods with an important extent of generality in describing the geometry and boundary conditions of the numerical methods. It calculates the temperature distribution in all involved borehole heat exchanger components and the surrounding soil mass using the fast Fourier transform, for the time domain; and the complex Fourier and Fourier-Bessel series, for the spatial domain. Numerical examples illustrating the model capability to reconstruct thermal response test data together with parametric analysis are given. The CPU time for calculating temperature distributions in all involved components, pipe-in, pipe-out, grout, and soil, using 16,384 FFT samples, for the time domain, and 100 Fourier-Bessel series samples, for the spatial domain, was in the order of 3 s in a normal PC. The model can be utilized for forward calculations of heat flow in a double U-tube geothermal heat pump system, and can be included in inverse calculations for parameter identification of shallow geothermal systems.

Cumulated energy consumption in a heat pump system using a U-tube ground heat exchanger in a moderate climate

This paper presents measurement results of a heating system equipped with a vapour compressor heat pump supplying a Polish single-family house with heat. The heat pump lower heat source is the rock mass where a vertical U-tube ground heat exchanger was installed. The measurements presented herein were performed between October 2011 and September 2013. The example measurement results illustrate the changes in the temperature of the medium at the ground heat exchanger inlet and outlet, the variability in the temperatures of the surroundings, as well as the variability in the heat pump operation time and electricity consumption in selected periods. The measurements made it possible to determine the analyzed system driving electric energy consumption throughout two heating seasons of identical duration in two subsequent years. The results allowed an analysis of the cumulated energy consumption in the periods mentioned above. The analysis was comparative in nature by relating to other selected heat sources.

A new RC and g-function hybrid model to simulate vertical ground heat exchangers

This paper presents a new hybrid two-dimensional model to simulate single U-tube ground heat exchangers based on the use of the electrical analogy to model heat transfer within the borehole and thermal response factors (short and long time-step g-functions) to estimate heat flow to the surrounding ground. Unlike other similar models, heat transfer equations are discretized only within the borehole domain, with two nodes for the fluid and two additional nodes for the grouting material, reducing the number of equations as ground temperature nodes are not considered. The substitution of ground nodes with short and long-term g-functions allows the simulation with short time steps keeping the possibility of simulating periods of several years. A complete analysis of the influence of characteristic parameters of the model in terms of the heat conduction path, vertical discretization level and equivalent heat capacity, has been carried out. The model shows a good performance for short simulation time-steps of five minutes for the prediction of both the short-term and long-term response. It has been successfully validated through comparison with a numerical computational fluid dynamic (CFD) reference model, achieving good RMSE values, which were smaller than 0.15 °C for water outlet temperature and borehole surface mean temperature.

Simulation study on the thermal performance of vertical U-tube heat exchangers for ground source heat pump system

The Vertical U-tube ground source heat exchanger (GSHE) is a significant component of the ground source heat pump (GSHP) system. This paper investigates the effect of Reynolds number, tube diameter and tube-connection configurations on the thermal and pressure performance of vertical U-tube GSHEs numerically. The simulation study was verified by ground thermal response experimental test. Meanwhile, the dimensionless evaluation criterion and the integrated evaluation criterion were adopted to analyze the thermal and pressure performance of GSHEs. The results show that the integrated evaluation factor was 5.11, 11.55, and 13.25 for Re = 100, Re = 1000, Re = 1600, respectively. The integrated evaluation factor was 11.48, 11.55 and 12.14 for different tube external diameters of DN25, DN32 and DN40. Therefore, better integrated performance can be obtained by increasing Reynolds number and U-tube diameter under laminar flow condition. Furthermore, parallel U-tubes GSHEs can ensure higher heat flux and lower pressure drop.

Light-Weight Design of the Tubesheet of a High Pressure U-Tube Heat Exchanger Based on Finite Element Analysis

Based on Chinese code GB151 for the design of shell-and-tube heat exchangers, the tubesheets are usually very heavy. The reason is attributed to the oversimplified mechanical model for the calculation of the tubesheet thickness especially the insufficient consideration of the tube support to the tubesheet. In this paper, a 3-D finite element model was established for a U-tube heat exchanger consisting of tubesheet, tube-channel, part of shell and tubes. A static stress analysis was performed under tube-side pressure loading. Optimal computation or light-weight design was carried out for minimum thickness of the tubesheet with the precondition of strength requirements conforming to the design-by-analysis code JB4732-1995. It is found that with the optimization design, the thickness of the tubesheet could be decreased by 31%, meaning that by using finite element method together with the code of design-by-analysis the thickness of the tubesheet can be significantly reduced.

Investigation of Trapezoidal-Cut Twisted Tape Insert in a Double Pipe U-Tube Heat Exchanger using Al2O3/Water Nanofluid

An attempt is made to enhance the rate of heat transfer in heat exchangers using Al2O3 nanofluid. In this work an experimental analysis on trapezoidal-cut twisted tape insert in a double pipe U-tube heat exchanger using Al2O3 water based nanofluid is presented. The heat transfer coefficients and the corresponding friction factors required for performance analysis are determined taking into account the typical operating conditions of the heat exchangers in turbulent flow regimes with particle volume concentration of 0.01% and 0.03% and twist ratios ranging between 5 and 20. Experimental data is generated at flow rates ranging from 0.0333kg/s to 0.2667kg/s. Experimental data is generated with water and nanofluid for Reynolds number in the range 3000<Re<30000, the Nusselt number of entire pipe for 0.03% concentrations of nanofluid with trapezoidal-cut twisted tape inserts of H/D = 5 is enhanced by 34.24% as compared to water. The friction factor of entire pipes for 0.03% concentration of nanofluid with trapezoidal-cut twisted tape inserts of H/D=5 is enhanced by 1.29 times as compared to water. The results of the investigation indicate an enhancement in the performance parameters of the heat exchanger namely heat transfer coefficient and friction factor with an increase in volume concentration of the nanoparticle.

Experimental study of heat transfer and friction factor of Al2O3 nanofluid in U-tube heat exchanger with helical tape inserts

Turbulent forced convection heat transfer and friction of Al2O3–water nanofluid flowing through a concentric tube U-bend heat exchanger with and without helical tape inserts in the inner tube were studied experimentally. The experiments were conducted in the Reynolds number range from 3000 to 30,000, volume concentrations of 0.01%, 0.03% and helical tape inserts of p/d=5, 10, 15 and 20. The results indicate that an increase in Reynolds number and Prandtl number yields to an increase in the average Nusselt number, and augmentation of thermal conductivity in the nanofluid contributes to heat transfer enhancement. The Nusselt number of entire pipes for 0.03% concentrations of nanofluid with helical tape inserts of p/d=5 shows an enhancement of 32.91%, as compared to water. The friction factor for the entire inner tube for 0.03% concentration of nanofluid with helical tape inserts of p/d=5 has increased by 1.38-times, as compared to water; in general and consistent with theory, the pressure drop in the inner tube increases with an increase in nanoparticle volume concentration and aspect ratio of the inserts. The empirical correlations for the Nusselt number and friction factor are obtained as functions of the Reynolds number, Prandtl number, volume concentration and aspect ratio.

New g-functions for the hourly simulation of double U-tube borehole heat exchanger fields

A new method for the hourly simulation of fields of long BHEs (borehole heat exchangers) is presented, in dimensionless form. The method considers also the internal structure of the BHE, with reference to double U-tube BHEs. The analytical expressions of new g-functions (time evolutions of the dimensionless temperature, averaged along the BHE length, due to a uniform and constant heat load) are presented, which complement those reported in Zanchini E., Lazzari S., Energy 2013; 59: 570–80. The new g-functions yield the dimensionless temperature at the interface tubes-grout, and allow a more precise determination of the time evolution of the temperature of the operating fluid during hourly peak loads. Moreover, g-functions at the interface tubes-grout and at several radial distances are reported also for BHEs with a ratio 500 between length and diameter, a value not considered in the previous paper. Due to its higher accuracy, the new method is recommended for the hourly simulation and the optimization of ground-coupled heap pump systems with double U-tube BHEs.

Performance investigation of a solar heating system with underground seasonal energy storage for greenhouse application

This study reports the performance of a demonstrated 2304 m2 solar-heated greenhouse equipped with a seasonal thermal energy storage system in Shanghai, east China. This energy storage system utilises 4970 m3 of underground soil to store the heat captured by a 500 m2 solar collector in non-heating seasons through U-tube heat exchangers. During heating seasons, thermal energy is delivered by the heat exchange tubes placed on the plants shelves and the bare soil. The system can operate without a heat pump, which can save electricity consumption and further enhance the solar fraction. It was found that in the first operation year, 331.9 GJ was charged, and 208.9 GJ was later extracted for greenhouse space heating. No auxiliary heating equipment was installed so that solar energy covered all the heating loads directly or indirectly. It was demonstrated that this system was capable of maintaining an interior air temperature that was 13 °C higher than the ambient value when the latter temperature was −2 °C at night. The ECOP (electrical coefficient of performance) of the first operation year was approximately 8.7, indicating a better performance than the common heat pump heating system.