Simulation and experimental analysis of optimal buried depth of the vertical U-tube ground heat exchanger for a ground-coupled heat pump system

This paper presents a numerical heat transfer model for vertical U-tube Ground Heat Exchangers (GHE). This model is uniquely able to take into to account different initial soil temperatures and physical properties at different depths. The model has been validated based on an experimental case study and has been used to simulate the thermal performance of GHEs. The simulation results show that for a 100-m vertical GHE, the first 70 m of the vertically buried GHE has a higher heat transfer capability than its last 30-m section. In addition, the validated model is used to investigate the optimal depth of vertical GHEs in five case studies ranging from 60 to 100 m length. Among them, the simulation results demonstrate that the GHE with buried depth of 70 m is able to provide the highest heat exchange rate per unit depth (54.1 and 47.0 W/m under heat rejection/extraction mode). It consequently results in shortest total GHE length of 11,388 m and the minimum cost of 1.82 million Yuan. However, a larger area is needed for boreholes and GHEs installation. Therefore, the optimal buried depth of the vertical U-tube GHEs for the studied case is 70 m on the condition of allocation of an abundant area to set up the boreholes.     

Heat transfer analysis of boreholes in vertical ground heat exchangers

A ground heat exchanger (GHE) is devised for extraction or injection of thermal energy from/into the ground. Bearing strong impact on GHE performance, the borehole thermal resistance is defined by the thermal properties of the construction materials and the arrangement of flow channels of the GHEs. Taking the fluid axial convective heat transfer and thermal “short-circuiting” among U-tube legs into account, a new quasi-three-dimensional model for vertical GHEs is established in this paper, which provides a better understanding of the heat transfer processes in the GHEs. Analytical solutions of the fluid temperature profiles along the borehole depth have been obtained. On this basis analytical expressions of the borehole resistance have been derived for different configurations of single and double U-tube boreholes. Then, different borehole configurations and flow circuit arrangements are assessed in regard to their borehole resistance. Calculations show that the double U-tubes boreholes are superior to those of the single U-tube with reduction in borehole resistance of 30-90%. And double U-tubes in parallel demonstrate better performance than those in series.     

Potential of geothermal heat exchangers for office building climatisation

Low depth geothermal heat exchangers can be efficiently used as a heat sink for building energy produced during summer. If annual average ambient temperatures are low enough, direct cooling of a building is possible. Alternatively the heat exchangers can replace cooling towers in combination with active cooling systems. In the current work, the performance of vertical and horizontal geothermal heat exchangers implemented in two office building climatisation projects is evaluated.A main result of the performance analysis is that the ground coupled heat exchangers have good coefficients of performance ranging from 13 to 20 as average annual ratios of cold produced to electricity used. Best performance is reached, if the ground cooling system is used to cool down high temperature ambient air. The maximum heat dissipation per meter of ground heat exchanger measured was lower than planned and varied between 8 W m^?1 for the low depth horizontal heat exchangers up to 25 W m^?1 for the vertical heat exchangers.The experimental results were used to validate a numerical simulation model, which was then used to study the influence of soil parameters and inlet temperatures to the ground heat exchangers. The power dissipation varies by ±30% depending on the soil conductivity. The heat conductivity of vertical tube filling material influences performance by another ±30% for different materials. Depending on the inlet temperature level to the ground heat exchanger, the dissipated power increases from 2 W m^?1 for direct cooling applications at 20 °C up to 52 W m^?1 for cooling tower substitutions at 40 °C. This directly influences the cooling costs, which vary between 0.12 and 2.8€ kW h^?1.As a result of the work, planning and operation recommendations for the optimal choice of ground coupled heat exchangers for office building cooling can be given.     

An average fluid temperature to estimate borehole thermal resistance of ground heat exchanger

Analysis of borehole thermal resistance is important for the standard sizing of ground heat exchanger (GHE). In this paper, a p-linear dimensionless average fluid temperature is proposed to estimate borehole thermal resistance. A p-linear dimensionless fluid temperature and p-linear dimensionless average fluid temperature are introduced, and the p-linear dimensionless fluid temperature is compared with theoretical dimensionless fluid temperature calculated by quasi-three-dimensional model for both single and double U-tubes. Results show that the p-linear dimensionless temperatures with parameters p → 0 and p = ?1/2 are respectively in good agreement with the theoretical dimensionless fluid temperatures of single and double U-tubes. Therefore, the dimensionless logarithmic mean temperature for p → 0 and the dimensionless geometric mean temperature for p = ?1/2 should respectively be adopted to reasonably estimate the thermal resistance of single and double U-tube boreholes.     

Exergoeconomic analysis of condenser type heat exchangers

In this study, an exergoeconomic analysis of condenser type parallel flow heat exchangers is presented. Exergy losses of the heat exchanger and investment and operation expenses related to this are determined with functions of steam mass flow rate and water exit temperature at constant values of thermal power of the heat exchanger at 75240 W, cold water mass flow rate and temperature. The inlet temperature of water is 18 °C and exit temperatures of water are varied from 25 °C to 36 °C. The values of temperature and pressure of saturated steam in the condenser are given to be T_con=47 ° C and P_con=10.53 kPa. Constant environment conditions are assumed. Annual operation hour and unit price of electrical energy are taken into account for determination of the annual operation expenses. Investment expenses are obtained according to the variation of heat capacity rate and logarithmic mean temperature difference and also heat exchanger dimension determined for each situation. The present analysis is hoped to be useful in determining the effective parameters for the most appropriate exergy losses together with operating conditions and in finding the optimum working points for the condenser type heat exchangers.     

Analysis for heat transfer enhancement of helical and electrical heating tube heat exchangers in vacuum freeze-drying plant

This paper investigates the heat transfer rate of the combined cooling-and-heating heat exchanger by using computational fluid dynamics (CFD) method. Several factors, such as additional baffles and heat transfer areas, are also discussed in order to improve the efficiency of heat exchanger in the vacuum freeze-drying system. The simulated result indicated that, for addition electrical heating tube, the heat transfer rate of the heat exchanger increased with the increasing length of the electrical heating tube. The increasing rates of secondary and primary drying stages were 2.774 and 2.986 W/mm, respectively. For additional vertical baffle, the variation of the heat transfer rate with respect to vertical baffle length was in the U-shape format. The minimum heat transfer rates of secondary drying, primary drying and freezing stages were 716.79 W and − 195.17 W and − 670.71 W, respectively. For additional W-shape vertical baffles, the heat transfer rate of this heat exchanger was maximum among these four designs. For the three stages of heat exchangers with these four designs, the shell side Nusselt number had the inverse linear relationship with the Reynolds number.     

Experimental analysis of flow and heat transfer in a miniature porous heat sink for high heat flux application

A novel miniature porous heat sink system was presented for dissipating high heat fluxes of electronic device, and its operational principle and characteristics were analyzed. The flow and heat transfer of miniature porous heat sink was experimentally investigated at high heat fluxes. It was observed that the heat load of up to 280 W (heat flux of 140 W/cm²) was removed by the heat sink with the coolant pressure drop of about 34 kPa across the heat sink system and the heater junction temperature of 62.9 °C at the coolant flow rate of 6.2 cm³/s. Nu number of heat sink increased with the increase of Re number, and maximum value of 323 for Nu was achieved at highest Re of 518. The overall heat transfer coefficient of heat sink increased with the increase of coolant flow rate and heat load, and the maximal heat transfer coefficient was 36.8 kW(m² °C)^?1 in the experiment. The minimum value of 0.16 °C/W for the whole thermal resistance of heat sink was achieved at flow rate of 6.2 cm³/s, and increasing coolant flow rate and heat fluxes could lead to the decrease in thermal resistance. The micro heat sink has good performance for electronics cooling at high heat fluxes, and it can improve the reliability and lifetime of electronic device.     

Assessment of an active-cooling micro-channel heat sink device,using electro-osmotic flow

Non-uniform heat flux generated by microchips causes “hot spots” in very small areas on the microchip surface. These hot spots are generated by the logic blocks in the microchip bay; however, memory blocks generate lower heat flux on contrast. The goal of this research is to design, fabricate, and test an active cooling micro-channel heat sink device that can operate under atmospheric pressure while achieving high-heat dissipation rate with a reduced chip-backside volume, particularly for spot cooling applications. An experimental setup was assembled and electro-osmotic flow (EOF) was used thus eliminating high pressure pumping system. A flow rate of 82 μL/min was achieved at 400 V of applied EOF voltage. An increase in the cooling fluid (buffer) temperature of 9.6 °C, 29.9 °C, 54.3 °C, and 80.1 °C was achieved for 0.4 W, 1.2 W, 2.1 W, and 4 W of heating powers, respectively. The substrate temperature at the middle of the microchannel was below 80.5 °C for all input power values. The maximum increase in the cooling fluid temperature due to the joule heating was 4.5 °C for 400 V of applied EOF voltage. Numerical calculations of temperatures and flow were conducted and the results were compared to experimental data. Nusselt number (Nu) for the 4 W case reached a maximum of 5.48 at the channel entrance and decreased to reach 4.56 for the rest of the channel. Nu number for EOF was about 10% higher when compared to the pressure driven flow. It was found that using a shorter channel length and an EOF voltage in the range of 400–600 V allows application of a heat flux in the order of 10⁴ W/m², applicable to spot cooling. For elevated voltages, the velocity due to EOF increased, leading to an increase in total heat transfer for a fixed duration of time; however, the joule heating also got elevated with increase in voltage.     

Experimental study of thermal behaviors in a rectangular channel with baffle of pores

This experimental study attempts to explore the local heat transfer in rectangular channel with baffles, and analyzes the experimental results of baffles with different heights and pores in the event of five Reynolds numbers and three heating quantities. Apart from increasing the perturbation of flow field, the channel's flow field with baffles, which is similar to a backward-facing step flow field, is very helpful to heat transfer. To obtain an optimized baffle and increase the perturbation of flow field, this experiment employed baffles with five heights (H = 10–50 mm) and different numbers of pores (N = 1–3), as well as heat flux: Q = 40–100 l/min, Reynolds number: 702–1752, and heating quantity: q_in = 90–750 W/m². In addition to measurement of overall temperature distribution, emphasis is also placed on analysis of local heat transfer coefficient. Furthermore, heat transfer distribution of channel can be applied to explain how the baffles of pores have an influence upon backward-facing step flow field, shear layer, recirculation region, reattachment region and redeveloped boundary layer. Finally, some empirical formulas derived form experimental results may provide a reference for future design.     

Improvement of micro-combustion stability through electrical heating

This experiment investigates the performance of a micro combustor under different operation conditions, and its improvement with electrical heating inside. According to the results, extinction and blow out occurred at 0.08 and 0.4 L/min, respectively. Electrical heating enhanced the stability effectively. With electrical heating power of 1.05 and 4.70 W, the equivalence ratio ranges of stable combustion are extended from 0.362 – 6.52 to 0.178 – 7.66 and 0.126 – 9.43 on average, respectively, and it is more effective in the lean and low-flow-rate cases. The micro flame shows obvious shift downstream at high flow rates. The flames in the lean cases have higher robustness, their reaction region are closer to the nozzle outlet around 1.17 mm on average than in the rich cases. The heat loss and heat conduction of the micro combustor surface are found to be related to its performance. The rich cases at moderate flow rates have 0.086 W lower heat loss on average than the lean cases, which enhances the stability effectively. However, the rich cases have 0.013 W lower heat conduction on average at moderate flow rates, thus more severe shift downstream occurs.     

Entropy generation of vapor condensation in the presence of a non-condensable gas in a shell and tube condenser

This article investigates the entropy production of condensation of a vapor in the presence of a non-condensable gas in a counter-current baffled shell and one-pass tube condenser. The non-dimensional entropy number is derived with respect to heat exchange between the bulk fluid and condensate, as well as heat exchange between the condensate and coolant. Numerical results show that heat transfer from the condensate to the coolant has a dominant role in generating entropy. For example, at an air mass flow rate of 330 kg/h, 93.4% of the total entropy generation is due to this source. The resultant profiles during the condensation process indicate that a higher air mass flow rate leads to a lower rate of entropy production. For example, as the air mass flow rate increases from 330 kg/h to 660 kg/h and 990 kg/h, the total entropy generation decreases from 976 J/s K to 904 and 857.2 J/s K, respectively. By introducing a new parameter called the condensation effectiveness, a correlation is also developed for predictions of the entropy number, and an illustrative example is presented.     

Investigation of natural convection induced outer side heat transfer rate of coiled-tube heat exchangers

Natural convection induced heat transfer has been studied over the outer surface of helically coiled-tube heat exchangers. Several different geometrical configurations (curvature ratio δ ε [0.035, 0.082]) and a wide range of flow parameters (60 <= T_tank <= 90, T_in = 19 and 60 <= T_in <= 90, T_tank = 20, 4000 <= Re <= 45000) have been examined to broaden the validity of the results gained from this research. A fluid-to-fluid boundary condition has been applied in the numerical calculations to create the most realistic flow configurations. Validity of the numerical calculations has been tested by experiments available in the open literature. Calculated results of the inner side heat transfer rate have also been compared to existing empirical formulas and experimental results to test the validity of the numerical computation in an independent way from the outer side validation of common helical tube heat exchangers. Water has been chosen to the working fluid inside and outside of the coiled tube (3 < Pr < 7). Outer side heat transfer rate along the helical tube axis has been investigated to get information about the performance of the heat transport process at different location of the helical tube. It was found that the outer side heat transfer rate is slightly dependent on the inner flow rate of any helical tube in case of increasing temperature differences between the tank working fluid temperature and the coil inlet temperature. A stable thermal boundary layer has been found along the axial direction of the tube.In addition to this the qualitative behavior of the peripherally averaged Nusselt number versus the axial location along the helical tube function is strongly dependent on the direction of the heat flow (from the tube to the storage tank and the reversed direction). Inner side heat transfer rate of helical coils have also been investigated in case of fluid-to-fluid boundary conditions and the calculation results have been compared with different prediction formulas published in the last couples of decades.     

竖直埋管地热换热器钻孔内的传热分析

准三维模型为竖直埋管地热换热器的结构优化提供了较为精确的理论基础。利用准三维模型对竖直埋管地热换热器进行分析与研究得出，不同的行程布置对双U型埋管地热换热器的传热性能有较大影响。就钻孔内热阻的对比，双U型埋管比单U型埋管钻孔内的热阻低，因而双U型埋管地热换热器较单U型埋管地热换热器更为合理。     

Multifunctional heat exchangers derived from three-dimensional micro-lattice structures

A micro-scale cross-flow heat exchanger is constructed from a hollow nickel micro-lattice structure, which is fabricated by conformally electroplating nickel onto a sacrificial polymer micro-lattice formed from self-propagating photopolymer waveguides. The periodic unit cell of the hollow nickel micro-lattice structure tested here includes lattice members with a diameter <1 mm and a nominal pore size <9 mm. The heat transfer performance of the micro-lattice-based heat exchanger is analyzed in terms of thermal conductance per unit volume, which is equal to the value of overall heat transfer coefficient multiplied by surface area to volume ratio. Calculated values range from 0.84 to 1.58 W/cm³K for Reynolds number ranges of between 3400 ± 200 and 6500 ± 500 for hot water flow inside the hollow lattice members and 85 ± 6 and 240 ± 20 for cold water flow around the lattice members. Based on a developed correlation, the experimental heat transfer data is used to predict the thermal performance of larger and smaller micro-lattice-based heat exchangers, as well as various micro-lattice feature dimensions that are tunable with the fabrication process (node-to-node spacing, inner diameter, etc.). The micro-lattice heat exchanger was tested under quasi-static compression and the results illustrate the multifunctional capability for load bearing and energy absorption applications. This work demonstrates a multifunctional heat exchanger with a fully-scalable fabrication process which is useful for size and weight constrained heat transfer applications, including those in the automotive and aerospace industries.     

Local heat transfer distribution and effect of instabilities during flow boiling in a silicon microchannel heat sink

Flow boiling of the perfluorinated dielectric fluid FC-77 in a silicon microchannel heat sink is investigated. The heat sink contains 60 parallel microchannels each of 100 μm width and 389 μm depth. Twenty-five evenly distributed temperature sensors in the substrate yield local heat transfer coefficients. The pressure drop across the channels is also measured. Experiments are conducted at five flow rates through the heat sink in the range of 20–80 ml/min with the inlet subcooling held at 26 K in all the tests. At each flow rate, the uniform heat input to the substrate is increased in steps so that the fluid experiences flow regimes from single-phase liquid flow to the occurrence of critical heat flux (CHF). In the upstream region of the channels, the flow develops from single-phase liquid flow at low heat fluxes to pulsating two-phase flow at high heat fluxes during flow instability that commences at a threshold heat flux in the range of 30.5–62.3 W/cm² depending on the flow rate. In the downstream region, progressive flow patterns from bubbly flow, slug flow, elongated bubbles or annular flow, alternating wispy-annular and churn flow, and wall dryout at highest heat fluxes are observed. As a result, the heat transfer coefficients in the downstream region experience substantial variations over the entire heat flux range, based on which five distinct boiling regimes are identified. In contrast, the heat transfer coefficient midway along the channels remains relatively constant over the heat flux range tested. Due to changes in flow patterns during flow instability, the heat transfer is enhanced both in the downstream region (prior to extended wall dryout) and in the upstream region. A previous study by the authors found no effect of instabilities during flow boiling in a heat sink with larger microchannels (each 300 μm wide and 389 μm deep); it appears therefore that the effect of instabilities on heat transfer is amplified in smaller-sized channels. While CHF increases with increasing flow rate, the pressure drop across the channels has only a minimal dependence on flow rate once boiling is initiated in the microchannels, and varies almost linearly with increasing heat flux.     

Temperature distributions in boreholes of a vertical ground-coupled heat pump system

The objective of this study is to show the temperature distribution development in the borehole of the ground-coupled heat pump systems (GCHPs) with time. The time interval for the study is 48 h. The vertical GCHP system using R-22 as refrigerant has a three single U-tube ground heat exchanger (GHE) made of polyethylene pipe with a 40 mm outside diameter. The GHE was placed in a vertical borehole (VB) with 30 (VB1), 60 (VB2) and 90 (VB3) m depths and 150 mm diameters. The experimental results were obtained in cooling and heating seasons of 2006–2007. A two-dimensional finite element model (FEM) was developed to simulate temperature distribution development in the soil surrounding the GHEs of GCHPs operating in the cooling and the heating modes. The finite element modelling of the GCHP system was performed using the ANSYS code. The FEM incorporated pipes, the grout and the surrounding formation. From the cases studied, this approach appears to be the most promising for estimation the temperature distribution response of GHEs to thermal loading.     

Heat transfer and pressure drop properties of high viscous solutions in plate heat exchangers

Heat transfer and pressure drop characteristics of an absorbent salt solution in a commercial plate heat exchanger serving as a solution sub-cooler in the high loop of triple-effect absorption refrigeration cycle was investigated. The main objectives of this research were to establish the correlation equations to predict the heat transfer and pressure drop and to analyze and optimize the operating parameters for use in the design of absorption systems.In order to conduct above studies, a single-pass cross-corrugated ALFA-LAVAL plate heat exchanger, Model PO1-VG, with capacity of 14,650 W (50,000 Btu/h) was used. In order to evaluate the performance, hot solution inlet temperatures from 55 °C (130 °F) to 77 °C (170 °F), and inlet temperature differences from 14 °C (25 °F) to 20 °C (35 °F) were used. The cold side of the heat exchanger was operated to match the equal heat capacity rate of hot side.Based on the empirical models proposed in the literature, a program was developed and experimental data were curve fitted. From the best-fitted curves, the power-law equations for heat transfer and pressure losses were established and the performance was evaluated.In the hot salt solution side, the Reynolds number was varied from 250 to 1100 and the resulting Nusselt number varied from 7.4 to 15.8. The measured overall heat transfer coefficient U_overall varied from 970 W/m² °C (170 Btu/h ft² °F) to 2270 W/m² °C (400 Btu/h ft² °F) and the Fanning friction factor in the absorbent side of the heat exchanger varied from 5.7 to 7.6. The correlation equations developed to predict the heat transfer and friction factor perfectly agree with the experimental results. Those equations can be used to predict the performance of any solution with Prandtl numbers between 82 and 174, for heat exchangers with similar geometry.     

Development of suitability maps for ground-coupled heat pump systems using groundwater and heat transport models

The thermophysical properties of subsurface materials (soils, sediments and rocks) and groundwater flow strongly affect the heat exchange rates of ground heat exchangers (GHEs). These rates can be maximized and the installation costs of the ground-coupled heat pump (GCHP) systems reduced by developing suitability maps based on local geological and hydrological information. Such maps were generated for the Chikushi Plain (western Japan) using field-survey data and a numerical modeling study. First, a field-wide groundwater model was developed for the area and the results matched against measured groundwater levels and vertical temperature profiles. Single GHE models were then constructed to simulate the heat exchange performance at different locations in the plain. Finally, suitability maps for GCHP systems were prepared using the results from the single GHE models. Variations in the heat exchange rates of over 40% revealed by the map were ascribed to differences in the GHE locations, confirming how important it is to use appropriate thermophysical data when designing GCHP systems.     

Measurements of ground temperatures in Cyprus for ground thermal applications

The knowledge of the thermal behaviour of the ground and the factors affecting it are important for many construction projects and for using the ground as heat storage or a heat exchange media. Not enough information exists regarding the thermal behaviour of the ground in Cyprus and this is one of the main interests of this study. The ground temperature distribution was recorded for the period between May 2006 and May 2007, at the Athalassa region in Nicosia. For this purpose a borehole was drilled and a U-tube heat exchanger made of 32 mm external diameter polyethylene pipe and thermocouples were placed up to the depth of 50 m. All data were recorded using an Omega OMB-DAQ 55/65 USB data acquisition module for a typical day each month at 15 min intervals. The surface zone in the area, reaches the depth of 0.5 m while the shallow zone extends up to 7 m. Below the 7 m depth the temperature remains almost unchanged at 22.6 °C and close to the mean annual ambient air temperature of 19.5 °C. Ground temperatures up to the depth of 7 m and 7.7 m were also recorded at the Ariel and Ayia Phyla locations in Limassol, respectively. The two boreholes were drilled to study and compare the thermal behaviour of the ground in other lowland places of the Island of Cyprus as well. The comparison of the data collected showed almost no differences between the three data collection points in the ground temperature distribution regardless of the differences in the composition of the ground.     

Heat transfer of a horizontal spiral heat exchanger under groundwater advection

Groundwater flows at approximately 1–3 m under the ground surface in a given region. If groundwater flow is present, the performance of a horizontal ground heat exchanger (HGHE), buried in a shallow trench, is enhanced. Nevertheless, owing to the general depth at which groundwater is present, research regarding the heat transfer of a ground heat exchanger (GHE) under conditions with groundwater flow has mainly focused on vertical GHE systems. To the authors’ knowledge, no such studies have addressed HGHEs. From a system design perspective, a prediction tool is needed to consider the groundwater flow, optimize the size of the horizontal heat exchanger, minimize the initial cost and maximize the operational efficiency. Therefore, in this study, a moving ring source model was established and solved analytically to describe the temperature response of a spiral heat exchanger with groundwater flow. In addition, experiments were carried out to study the soil temperature variation during the operation of a spiral heater with different water velocities. The validity of the proposed model was proven by the good agreement between the experimental and calculated results. The average virtual tube surface temperature variations of single ring sources in two different configurations are discussed. Furthermore, the average virtual tube surface temperatures of multiple ring sources extending from single arrangements were computed and approximation algorithms were introduced to reduce the calculation time. The approximation approach has been proven to run thousands of times faster than the initial method, and the calculation results are in 97% agreement with those of the initial method. In summary, this study provides a useful tool for the design of spiral heat exchangers.