A 3-Dimensional Numerical Thermal Analysis for A Vertical Double U-Tube Ground-Coupled Heat Pump

Ali H Tarrad,Université De Lorraine, Cnrs, Lemta, Nancy, France

doi:10.18178/ijcea.2021.12.2.789

Ali H Tarrad, Université De Lorraine, Cnrs, Lemta, Nancy, France

Open Access

https://doi.org/10.18178/ijcea.2021.12.2.789

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Abstract

The ground heat exchanger plays a major role in the thermal performance and economic optimization of the ground-coupled heat pump. The present study focuses on the effect of the borehole size and the grout and soil thermal properties on the thermal assessment of these heat exchangers. A double U-tube heat exchanger was studied numerically by the COMSOL Multiphysics 5.4 software in a 3-dimensional discretization model. The double U-tube was circuited as a parallel flow arrangement and situated in a parallel configuration (PFPD) deep in the borehole. The grout and ground thermal conductivities were selected in the range of (0.73-2.0) W/m.K and (1.24-2.8) W/m.K respectively. The results revealed that the ground thermal conductivity showed a more pronounced influence on the thermal performance of the ground heat exchanger and with less extent for the grouting one. Increasing the grout filling thermal conductivity from (0.73) W/m.K to (2.0) W/m.K at a fixed ground thermal conductivity of (2.4) W/m.K has augmented the heat transfer rate by (10) %. The heat transfer rate of the ground heat exchanger exhibited marked enhancement as much as double when the ground thermal conductivity was increased from (1.24) W/m.K to (2.8) W/m.K at fixed grout thermal conductivity range of (0.78-2.0) W/m.K. It has been verified that increasing the borehole size has a negligible effect on the ground heat exchanger thermal performance when a grout with a high thermal conductivity was utilized in the ranged of examined configurations. The steady-state numerical analysis model outcomes of the present work could be implemented for the preliminary borehole design for a ground heat exchanger.

Highlights

The understanding of such topics of heat transfer took a deep consideration in the experimental, analytical, and numerical research by scientists, Ingersoll et al [1], Carslaw and Jaeger [2], Kavanaugh [3], Zeng et al [4], and Muttil and Chau [5]
Zanchini et al [16], [17] utilized the COMSOL Multiphysics 3.4 software to study the effects of flow direction and thermal shortcircuiting on the performance of small and 100 m long coaxial ground heat exchangers
A 3-dimensional model was built by the implementation of the COMSOL Multiphysics 5.4 software

Summary

INTRODUCTION

The understanding of such topics of heat transfer took a deep consideration in the experimental, analytical, and numerical research by scientists, Ingersoll et al [1], Carslaw and Jaeger [2], Kavanaugh [3], Zeng et al [4], and Muttil and Chau [5]. A 2-dimensional time-dependent numerical model was accomplished to consider the heat flow in the ground by Zeng and Fang [13] and Zeng et al [14]. This was because the temperature variation inside the borehole is usually slow and minor. A 3-dimensional model was built by the implementation of the COMSOL Multiphysics 5.4 software He concluded that the heat transfer rate of the double U-tube was better than that of the single one by (10-14) % when operates at the same total fluid mass flow rate and inlet temperature for a given borehole design. A steady-state 3-dimensional mode for the ground U-tube heat exchanger accomplished by the COMSOL Multiphysics 5.4 software [19] is presented

Materials and Thermal Properties

Mathematical Representation

DATA REDUCTION

RESULTS AND DISCUSSION

Borehole Size

CONCLUSION