Heat Transfer In Pipe Research Articles

Study on Heat Transfer Performance and Parameter Improvement of Gravity-Assisted Heat Pipe Heat Transfer Unit for Waste Heat Recovery from Mine Return Air

One of the effective methods for energy conservation and emission reduction in coal mines is to utilize waste heat recovery technology to recover mine return air waste heat. The gravity heat pipe is widely used in mine return air waste heat recovery due to its sustainable and economic advantages, but its heat transfer is a complex process influenced by multiple parameters. A single-tube heat transfer resistance model and a heat transfer calculation model based on enthalpy difference were established for the heat exchange tubes. Four typical application cases of a low flow rate and a low number of tube rows were selected, and their heat transfer characteristics were tested onsite and analyzed. It was found that there were problems such as a low overall heat transfer efficiency, a low fresh air outlet temperature, and a risk of icing in the final tube section. The effects of the gravity heat pipe parameters on the heat transfer performance were studied, such as the tube outer diameter, tube spacing, and the finned tube outer diameter. It was found that the air-resistant force of the heat exchanger increased with the increase of the tube spacing and the finned tube outer diameter, the heat transfer resistance increased with the increase of the tube spacing and the decrease of the finned tube outer diameter, and the heat transfer coefficient first increased and then decreased with the increase of the tube outer diameter. A configuration improvement scheme with a high flow rate and a high number of tube rows is proposed here. Taking Case 2 as an example, the temperature distribution of the heat tube before and after improvement is compared and analyzed. The results show that the heat transfer performance of the heat exchange system significantly improved. Without increasing the air resistance of the heat tube, the temperature of the return air outlet after improvement was reduced to 1.1 °C, 4.1 °C lower than that before improvement, further recovering the waste heat of the mine return air. The temperature of the condensate water film was greater than 0.5 °C, avoiding the icing problem of the condensate tube section, the fresh air outlet temperature reached 5.2 °C, an increase of 7.8 °C compared to that before improvement, and the overall heat transfer efficiency increased from 56.7% to 66%.

Analysis of Liquid Film Evaporation in Porous Particles: Toward Optimal Wick Parameters for Heat Transfer in Heat Pipes

Abstract Evaporation of working fluids inside capillary wicks determines the heat transfer capability of heat pipes. However, the relationship between wick parameters and evaporative heat transfer remains unclear. To establish a correlation between wick parameters of sintered porous particles and evaporation characteristics, a boundary condition model was developed, incorporating wick parameters such as particle radius (R), particle distance (d), apparent contact angle (θa), and initial liquid height (H). In the absence of a significant size effect, the profile of the liquid–vapor interface was determined using the boundary model by numerically solving the augmented Young–Laplace equation. Ammonia was used as an example to investigate evaporation characteristics. The curvature radius of the intrinsic meniscus (Re) was found to serve as a bridging factor between these wick parameters and evaporation characteristics. When Re exceeded 40.3 μm, a limitation in evaporative heat transfer within the thin film region was observed. The relationship between R, d, θa, and H was quantitatively described based on this evaporative heat transfer limit. Furthermore, a nondimensional analysis of the governing equation for the evaporating liquid film profile was conducted, yielding an influencing factor (λ) that governed the thin film profile. The proposed model and its outcomes could offer valuable theoretical insights for the structural design of sintered porous particles, the optimization of surface modification levels, and the determination of the appropriate working fluid charging ratio during the manufacturing process of heat pipes.

Numerical analysis of the heat transfer enhancement by using metal foam

Forced convection heat transfer in metal foam pipe under uniform heat flux was investigated numerically. Water has been used as the fluid medium in numerical simulations with copper metal foam. The test sample used metal foam for four cases (A, B, C, and D) by using varying thicknesses of foam along the pipe. In case (A), the pipe was completely filled with metal foam; in case (B), the pipe was partially embedded with metal foam and close to the wall; in case (C), the pipe was partially filled in the section's center; and in case (D), the pipe was partially filled in the form of three sections along the pipe. The foam employed in this study has a porosity of 0.90 and pore density ranges from (10–40) PPI. The fluid and energy transfers are simulated using the Forchheimer-extended Darcy equation and the local thermal non-equilibrium (LTNE) model. The simulation is performed using the commercial software FLUENT with specific boundary conditions. Turbulence was taken into account using the k- ε model. The results reveal that when the pipe is filled partially with metallic foam as indicated in case B, the Nusselt ratio rises due to the velocity gradient and temperature distribution is more homogenous inside the pipe led to enhancement in overall heat transfer. Also, the metallic foam 40PPI has a highly Nusselt number ratio, with a big value equal to 5.5 in case B due to the highly conductive surface area of heat transfer from the wall heated to the fluid. Additionally, the pipe in case A was completely filled with metallic foam, resulting in a bigger pressure drop than in the other cases, which increases flow resistance. The metal foam in case C, which is centered in the pipe's center, has the lowest pressure drop measurement. The 10PPI metallic foam exhibits the highest performance factor when compared to other metallic foam.

Effect of evaporation and condensation section length ratio on thermal performance of aluminum flat plate heat pipe with different micro grooved wicks

The length of the evaporation or condensation section has a significant impact on conventional heat pipe or copper flat heat pipe heat transfer performance. Based on this demand, a novel concept of the length ratio of evaporation and condensation section (LREC) of the flat plate micro grooved heat pipe (FPMHP) was proposed and the effect of the LRECs on the thermal performance of aluminum-based FPMHP with different micro grooved wicks was experimentally investigated for the first time. The experiment explored the effects of nine different LRECs on the thermal performance of FPMHP with the same section size, filling mass, rectangular and triangular micro grooved wicks installed at 90°, a constantly increasing input heat flux, and forced air cooling. The findings demonstrate that rectangular micro grooved wick flat heat pipe (FPMHP-r) and triangular grooved wick flat heat pipe (FPMHP-t) have the shortest startup times when LREC is 1.0, with FPMHP-r starting 70 s sooner than FPMHP-t. When 0.8 ≤ LREC ≤ 1.25, the heat transfer performance of FPMHP-t and FPMHP-r is excellent, better than performance when LREC < 0.8 or LREC > 1.25, and the LREC is recommended to be 0.8 ∼ 1.25 in practical applications. The optimal LREC value of aluminum-based FPMHP is 1.0, which can maximizes the heat transfer performance. Under the same conditions, the heat transfer performance of FPMHP-r is better than FPMHP-t, flat heat pipe wicks with high permeability, low porosity and effective pore radius should be designed in practice. The results can provide a theoretical basis for the design and optimization of aluminum-based FPMHP application.

Evaluation of heating capacity of shallow ground heat exchangers in different Locations– A case study of two administrative districts of Xi'an

As a case study, two administrative districts of Xi'an are selected. Eight sites are drilled for shallow ground heat exchangers (SGHEs), and thermal response tests (TRTs) are conducted on the ground to investigate the heating capacity of SGHEs in different locations. The vertical distribution of initial ground temperature is identified using a temperature sensing system in TRTs. Simultaneously, the real-time inlet and outlet water temperatures of SGHEs are monitored by controlling heating power and buried pipe flow rate to be constant, and the thermophysical parameters of the ground are derived. A three-dimensional full-scale numerical model for buried pipe heat transfer is developed based on the results of TRTs. The heating capacity distribution of SGHEs is determined by clarifying the characteristics of buried pipe heat transfer at various locations and depths. The results showed that the maximum difference in geothermal gradient and ground thermal conductivity is 51.2 °C/km and 0.51 W/(m·K) respectively, indicating that the ground parameters vary significantly within the chosen places. The heat transfer intensity of SGHEs is directly related to buried pipe depth and mean circulating water temperature at the same locations. The same prediction formula can describe the heat transfer intensity of SGHEs at various locations.

Capillary limit of a sodium screen-wick heat pipe

High-temperature alkali-metal heat pipes are passive heat transfer devices, where the capillary pressure created by the menisci in the wick pumps the condensed fluid back to the evaporator. Thus, the heat pipe heat transfer capacity is limited by the maximum capillary pressure at high heat fluxes. This work presents a two-dimensional heat and mass transfer model for alkali metal heat pipes. In addition to the traditional wall heat conduction model, the model also incorporates gas–liquid flow heat transfer and capillary structure models, enabling the model to simulate both steady-state operation and transient capillary limit processes. The predictions of the model agreed well with experimental data. The predicted critical heat flux temperature has an error of less than 20℃ and a relative error of 6%. The predicted transient sequences are consistent with the measurements. The model shows that capillary limit is affected by inclination angle, heat flux, and wick structure. For inclinations, the critical heat flux at the capillary limit is linearly related to the inclination angle with the predicted temperature rising 2–10℃/s during the early stage of the capillary limit critical heat flux condition. The evaporator temperature oscillates before the capillary limit with positive inclinations. For heat fluxes, increasing the heat flux by 17% increases the temperature increase rate 10 fold with a longer dryout length. The evaporator exhibits hysteresis after the capillary limit due to partial dryout of evaporator. For wick structures, reducing the mesh screen pitch from 80 μm to 15 μm reduces the critical heat flux to 10% of the CHF at 80 μm. This model can accurately predict the heat pipe temperature variations at the capillary limit and can explain the physical mechanisms for the dryout oscillations and recovery hysteresis at the capillary limit. Thus, the measurements and model provide analytical guidelines for designing heat pipe cooling systems.

THE STUDY OF HEAT TRANSFER INTENSITY AND AERODYNAMIC DRAG INSIDE A FLAT PIPE

It is know that a heat exchange tube with a flat-oval profile has excellent thermal and aerodynamic characteristics in contrast to a round tube. Thus, with the same planes, a flat tube has a significant increase in thermal and aerodynamic efficiency compared to a round tube, which is widely used in industry. However, at present, there are a limited number of publications on the study of heat transfer and aerodynamics inside a flat pipe.&#x0D; In this paper, we present the methodology and results of the study of heat transfer and aerodynamic drag in a pipe, and describe the experimental stand for such studies.&#x0D; The experiments performed in a flowing wind tunnel with an internal diameter of 36 mm, operating in an open-loop circuit. The working medium is air drawn from the laboratory room. The prototype was a steel pipe with a flat-oval profile, 320 mm long, 30x15 mm cross section, and 2 mm wall thickness.&#x0D; The created experimental stand allows us to study the heat transfer and aerodynamic drag of a flat pipe under the boundary condition q=const. An electric heater ensured this condition, which is a nichrome wire with a diameter of 0.6 mm wound along the entire length of the pipe and insulated from the external environment.&#x0D; The experiments were carried out in the range of Reynolds numbers (10.5 - 55.0) 103 and dissipated powers (50 - 150) W. The average air temperature in the pipe was set in the range (20 - 55) 0C, and the average temperature of the pipe wall was set in the range (24 - 140) 0C in accordance with the electric power supplied to the heater.&#x0D; Empirical correlations proposed for determining the intensity of heat transfer and aerodynamic drag inside a flat pipe. The data had compared with a round tube. The results had analyzed, and it had shown for the first time that the intensity of heat transfer and aerodynamic drag in a flat-oval pipe is 1.1 to 1.2 times and 1.4 to 1.7 times higher, respectively.

A comparative study on thermohydraulic performance of multi-channel spoilers and staggered spoilers

Energy crisis has set back the global progress on economic development, because the non-renewable energy sources are unsustainable, and renewable energy sources are not yet reliable. Therefore, improving the efficiency of energy usage becomes increasingly significant. Heat is the largest energy end-use in the world, taking good advantage of it is one of the most important subjects. Tubular heat exchangers are widely used in both civil and industrial processes, and the consumption of energy can be greatly reduced by improving the efficiency of heat transfer in pipes. Adding inserts into the pipes is a main way to enhance heat transfer. This paper mainly studies two new types of spiral spoilers, namely multi-channel spoilers and staggered spoilers, using numerical methods. The former divides the pipe into several channels, and the latter has deflecting angles at the end of every spiral period. Experiments were also conducted to verify the numerical results. Aiming at analyzing thermohydraulic characteristics of fluid flow in pipes inserted with two types of spoilers separately, the effects of channel number (N), deflecting angle (φ), pitch ratio (p’/D) were studied under different Reynolds numbers (Re). Evaluation parameters including Nusselt number (Nu), friction factor (f), heat deficit loss (eT), mechanical deficit loss (ep), exergy loss (E*) and performance evaluation criteria (PEC) were all considered as indices. Results show that flow channels and deflecting angles generate better mixed fluid, which is conducive to heat transfer enhancement at the expense of more irreversible loss. Multi-channel spoilers with a channel number N = 2 and pitch ratio p’/D = 1.5 maintain the best balance between energy loss and heat transfer. The heat transfer enhancement reaches the best performance in staggered spoilers with a deflecting angle φ = 15°, 45.07% irreversible loss can be reduced compared with φ = 120°. Staggered spoilers can enhance heat transfer better, compared with multi-channel spoilers, especially in laminar flow.

Thermal performance analysis of heat transfer in pipe by using metal foam

Thermal performance analysis of heat transfer in pipe by using metal foam

A study of the thermal performance prediction of embedded pipes in protection engineering based on multiple regression analysis

There are currently a large number of information equipment and similar devices that generate a significant amount of residual heat that must be eliminated through protection engineering. The ground heat exchanger can be useful in the treatment of residual heat due to their high efficiency and adequate concealment. In order to eliminate internal waste heat, the heat exchanger are laid within the inner surface of the surrounding soil at engineering entrances. In this study, the numerical heat transfer model of embedded pipes in protection engineering is developed based on the finite element software COMSOL. In addition, a total of eleven independent variables representing influencing factors of embedded pipe heat transfer were taken into account. The average heat transfer rate per meter, the heat exchanger effectiveness, and the total thermal resistance in the first 20 days of embedded pipe heat transfer were taken into account as dependent variables. Accordingly, a single factor regression model was developed in order to investigate the correlation between the three dependent variables and each independent variable, and the independent variables that had no significant influence on the dependent variables were removed. Then, the orthogonal experiment tables of 10 factor 5 level, 7 factor 5 level, and 4 factor 5 level were established based on the three dependent variables and independent variables, respectively. According to the experimental groups in the three tables, the step-by-step method in multiple regression analysis was employed in order to obtain the prediction formula of the average heat transfer rate per meter, the heat exchanger effectiveness, and the total thermal resistance in the first 20 days of embedded pipe heat transfer. At the same time, the influence degree of each independent variable on the dependent variables is obtained. Three evaluation indexes were selected to analyze the error of prediction results and numerical solutions. It was found that the prediction model was accurate overall. Furthermore, the prediction formula is compared to the analytical solution, and the results obtained by the two methods are comparable. The predictive solution is superior to the analytical solution because of the fewer simplified conditions and a lot of numerical simulation. These results indicate that, it is worth noting that the analysis of the influence of each factor and the predictive solution have a certain reference value for the future application of embedded pipes.

Unsteady heat transfer characteristics of R-407C in the subcooled flow boiling stemming from the oscillating refrigerant flow rate

The objective of this experiment is to investigate the unsteady heat transfer of R-407C subcooled flow boiling (SCFB) due to the oscillating refrigerant flow rate within a narrow tube oriented horizontally for application in energy efficiency air conditioning system. The refrigerant flow rate is input in the cycle of triangular oscillations. Influences of oscillation amplitude (ΔG/G‾) and oscillation period (tp) of the periodic flow rate on SCFB heat transfer are studied. The present results disclose that either ΔG/G‾ or tp has minor impact on time-average SCFB heat transfer performance. However, obvious changes in the temporal variations of the wall temperature on the pipe (Tw) and convective heat transfer coefficient (hr) are found, leading to three heat transfer mechanisms of single phase convection, intermittent nucleate boiling, and persistent subcooled boiling for various input heat fluxes. The Tw of persistent subcooled boiling presents the reversed tendency compared to that of single phase convection due to the variation of the triangular R-407C oscillation. In the case of intermediate heat flux, the unusual boiling phenomenon, called intermittent nucleate boiling, can be obtained depending on the R-407C mass flux. The visualized bubble images are also provided to further describe the intermittent nucleate boiling.

Comparison of the experimental and numerical results of coaxial-type and U-type deep-buried pipes’ heat transfer performances

The ground heat exchanger (GHE) is one of the main forms of geothermal energy utilization in middle-deep strata and typically includes coaxial-type deep-buried GHE (C-DGHE) and U-type deep-buried GHE (U-DGHE). This study compares the heat transfer characteristics of C-DGHE and U-DGHE. A field experiment was carried out on C-DGHE and U-DGHE at 2539 and 2781 m depths, respectively, in a housing estate in Xi'an. Simultaneously, the full-scale numerical models of both C-DGHE and U-DGHE at 2781 m were established based on the field logging data. A short-term experiment (72 h) and a long-term heat transfer simulation (five heating periods) serve to comprehensively compare the heat transfer performances of the two deep-buried pipe types. Results demonstrate that the heat transfer amount per unit meter of C-DGHE and U-DGHE were 132.18–145.91 W/m and 111.15–140.76 W/m respectively in this study, which means C-DGHE is superior to U-DGHE under similar boundary conditions when only heat transfer in buried pipes is considered. However, when heat transfer in buried pipes and water pump power consumption are considered simultaneously, a critical G (GC was ∼8.63 kg/s in this study) makes the comprehensive heat transfer performance of U-DGHE better than that of C-DGHE.

Turbulent heat transfer in pipes with increased roughness through shavings of helical ribs

Large-eddy simulations of mircrostructured pipes are carried out in this study. The pipes are helically ribbed with several starts from which shavings are cut out along the rib. A Reynolds number of 16,000 and a Prandtl number of 9 are applied. Cyclic boundary conditions are used to simulate an infinitely long pipe and a LES turbulence model is applied. The results show that both the number of starts and the distance of the shavings along the rib, have an impact on turbulence and heat transfer. An optimum was found for the number of starts regarding heat transfer. The heat transfer area increases for a higher number of starts, but the flow-related heat transfer decreases. Thus, an optimal arrangement of starts was identified for this pipe configuration. The variation of the arrangement showed that the pressure loss can be reduced by an inline configuration, whereby the heat transfer is only slightly decreased. It can be shown that the simulations can contribute to optimising complex structures in the future and thus accelerate the development process of new geometries. It can be shown that the development of new heat exchanger pipes is also possible with complex structures, this can be demonstrated here for the first time for structures that can actually be fabricated.

Heat transfer enhancement in a tube with a twisted tape inserted for various length ratios

ABSTRACT Energy efficiency is a major concern in pipe systems, particularly in applications where heat transfer is significant. Twisted tape insertion is used to enhance heat transfer in pipes. In this numerical study, twisted tapes with various length ratios (LR) (30–100%) were inserted into a tube filled with water. The tube surface was subjected to a uniform heat flux. The Nusselt number (Nu) and friction factor (f) variations were investigated for several Reynolds number values (Re) (2500–10,000) under hydrodynamically developed and thermally developing flow conditions between laminar and turbulent flows in the transition and low-turbulent regime. The results are validated with the available theoretical results and experiments. The maximum Nu enhancement is observed at an LR of 90% based on a factor of 1.55; an LR of 70% demonstrated a similar factor of ~1.5 and a friction factor 3.7 times more than that of plain tubes. This ratio provides the best results for the enhancement factor, which is 1.2 times higher at the maximum level. Consequently, an LR of 70% is considered to be sufficient to obtain an optimum combination of maximum heat transfer and minimum friction combination, and this combination is preferred over completely filling the tube with twisted tape.

Research on Heat and Mass Transfer Performance of Corrugated Coils in Closed Cooling Towers

According to the characteristics of the flow in the corrugated pipe, Fluent is used to simulate the 3D numerical simulation of the fluid in the corrugated pipe and the ordinary straight pipe by using the RNG k-ε model. Water at a certain temperature is used as the medium. The flow velocity is 0.5 m/s, 0.75 m/s, 1 m/s, 1.25 m/s, 1.5 m/s, in-pipe characteristics with a pipe diameter of 25mm and a pipe length of 1030mm. The simulation adopts the unstructured hexahedral mesh in the simulation, and the SIMPLE algorithm is used for pressure correction; the algebraic equation is iteratively solved by the over-relaxation method. The temperature and velocity at different cross-sections when the flow and heat exchange enter the fully developed stage are obtained through simulation calculation, and they are compared with the related characteristics of flow and heat transfer in the light pipe. The cooling tower of the straight pipe is tested, and the results show that the heat transfer performance of the corrugated pipe is better than that of the ordinary straight pipe.

Experimental study on sodium Screen-Wick heat pipe capillary limit

Heat pipes continuously transport the latent heat of vaporization from the evaporator to the condenser in the heat pipes due to the capillary pressure created by the menisci in the wick pumping the condensed fluid back to the evaporator. The heat pipe heat transfer capacity is then limited by the capillary limit. However, fewer capillary limit experiments were conducted to investigate the transient temperature variations and physical mechanism at the capillary limit for liquid metal wick heat pipes. This work presents capillary limit transient measurements on a sodium screen-wick heat pipe for high heat fluxes, high heating rates during frozen startup, and negative and positive inclination angles. It also investigates the special behavior before and after dryout, including dryout oscillations and the recovery process to understand the cause and physical mechanism at the capillary limit. The direct cause of the capillary limit is that the evaporation rate exceeds the capillary-driven liquid supply to the evaporator. The experiments show that the capillary limit creates a rapid temperature increase of about 5 °C/s at the evaporator end, while the condenser and adiabatic section temperatures decrease. Dryout oscillations were observed with positive inclinations before the capillary limit. After the capillary limit, reducing the power to just below the limit does not lead to heat pipe recovery, but the power needs to be reduced by about 30% of the critical heat flux to make the heat pipe return to its normal operating state.

Using a heat pipe as a waste air heat exchanger in a central air conditioner.

The purpose of this work is to analyze the literature on heat pipes to make a decision on the feasibility of using this type of heat exchanger in air conditioning systems. The article describes the principle of operation and design of the heat pipe, provides various ways to return condensate, and also describes in detail its thermodynamic cycle. Considerable attention is paid to various wick structures and working fluids. The following are the restrictions on heat transfer of heat pipes that affect the performance of the heat exchanger. Design recommendations found in the scientific literature and on the websites of manufacturing companies are also described. The diagrams of the heat pipe designs used in central air conditioners and in AC system are given. An example of an installation using heat pipes as a waste air heat exchanger in a central air conditioner is shown. Further, the efficiency and thermal power of various methods of heat recovery are compared with the same initial parameters of the flow rate and temperatures of outdoor, exhaust air. In conclusion, the conclusion is made about the advantages of using heat pipes in AC system for heat recovery. The reason for the rare use of this heat exchanger is also explained. The result of comparing different methods of heat recovery and the answer to the question about the prospects of using a heat pipe in the AC system is given. Recommendations are given on the direction of further research in this topic.

Analytic solutions of heat and mass transfer in flat heat pipes with porous wicks

Flat heat pipes have been used in cooling computer chips. However, previous models impose heat rate through the pipe as an input parameter, preventing optimization of heat transfer. We consider a horizontal flat heat pipe with an idealized porous wick on either one or both walls. A constant temperature difference is imposed across the two ends of the pipe, and we seek a steady-state solution. The pores in the wick are straight circular capillaries running along and across the wick, and are filled with a partially-wetting liquid. Its vapor fills the center region of the pipe and is connected to the liquid through circular pore openings on the wick surface. At the hot end, the liquid evaporates from the pore openings into vapor, which moves to the cold end where it condenses and the condensate flows in the wick to the hot end to complete a cycle. The mass evaporation rate at each pore opening on the wick surface is found from evaporation kinetics. This pore-level result is incorporated into pipe-level mass, momentum, and energy balances. The mass evaporation rate is found to depend only on the pipe temperature and is essentially independent of the vapor pressure. Consequently, the pipe behaves like a fin with evaporative cooling instead of convective cooling. Thus, the analytic fin solution holds for the pipe temperature and contains only one dimensionless number S (S2 is the ratio of evaporative to conductive heat rate through the pipe). It is observed that in heat pipes, S≫1 and thermal energy is chiefly transferred by vapor flow. Analytic solutions are subsequently found for the evaporation rate, the heat rate through the pipe, and the liquid and vapor flows along the pipe. Our idealized-wick parameters are converted to the porosity and permeability of a porous medium, so that we can compare directly with two published flat heat-pipe experiments. We analyze the effects of pipe length and wick thickness on the heat rate through the pipe, and provide guides for future design of flat heat pipes.

Heat exchange pipe spacing for optimal temperature uniformity on cold radiant ceiling surfaces

Radiant cooling systems have great potential for comfort and energy savings, but uneven surface temperature distributions and the risk of condensation restrict their use in engineering applications. In this study, a cold radiant panel heat transfer model was established and experimentally verified. Radiant panels with 3 heat transfer pipe spacings were designed and used in 7 tandem combination modules with variable pipe spacing (VPS) that were compared with 3 conventional cold radiant panel tandem combination modules with equal pipe spacing (EPS). Compared to the EPS modules, the overall heat transfer performance of the VPS modules was better, the surface temperature distribution was more uniform, and the average surface temperature was lower for different values of the operating parameters such as the water supply flow rate and supply and return water temperature difference. Additionally, the values of the temperature nonuniform coefficient (TNC) and surface average temperature were lower for the VPS than the EPS modules. Although the method of combining cold radiant panels with different heat transfer pipe spacings in series to form a cooling module improves the overall heat transfer performance, the balance of cost and heat transfer performance improvement should be considered in practical applications.

Study on Calculation Method of Heat Exchange Capacity and Thermal Properties of Buried Pipes in the Fractured Rock Mass-Taking a Project in Carbonate Rock Area as an Example

Fractures are developed in carbonate rock areas, and the fracture water flow significantly influences the heat exchange between buried pipes and the rock mass by induing heat convection, providing the carbonate rock area a strong heat exchange capacity and preferable conditions for shallow geothermal development and utilization. In this paper, the calculation method of heat exchange capacity of buried pipes based on fracture distribution characteristics is proposed and deduced, featuring such advantages as quick speed and low cost. Taking an actual project in carbonate rock area as an example, the heat exchange capacity of buried pipes was obtained by the following two methods: in-situ thermal response test and calculation based on fracture distribution characteristics. In the thermal response test, the initial ground temperatures of the two test holes were 15.18 °C and 12.72 °C. By fitting the linear equation of time and average temperature with a linear thermal source model, the heat exchange capacities were 57.21 W/m and 58.22 W/m, the thermal conductivities were 3.56 W/(m·K) and 2.32 W/(m·K), the thermal diffusivities were 1.71 × 10−6 m2/s and 1.12 × 10−6 m2/s, and the volume specific heat capacity was 2.08 × 106 J (m3·K). The test results indicated that the thermal property parameters of rock and soil mass were higher than those of other areas, with obvious wide-range distribution characteristics. Through the statistical analysis of outcrop fracture characteristics, combined with the cube law to calculate the fracture water flow and convective heat transfer, an alternative method for the calculation and optimization of buried pipe heat transfer in fractured rock mass area is also proposed in this paper. According to the measured fracture distribution characteristics of the field outcrop, the heat exchange capacities of the two holes were 57.26 W/m and 58.56 W/m, which were basically consistent with the thermal response test values and verified the reliability of the calculation method of heat exchange capacity of buried pipes based on fracture distribution characteristics.