Open Access

ARTICLE

Analytical Modeling of Internal Thermal Mass: Transient Heat Conduction in a Sphere under Constant, Exponential, and Periodic Ambient Temperatures

Liangjian Lei^1,2, Yihang Lu^1,2,*

1 The College of Electrical Engineering, Zhejiang University of Water Resources and Electric Power, Hangzhou, 310018, China
2 Zhejiang-Belarus Joint Laboratory of Intelligent Equipment and System for Water Conservancy and Hydropower Safety Monitoring, Hangzhou, 310018, China

* Corresponding Author: Yihang Lu. Email:

(This article belongs to the Special Issue: Heat Transfer Analysis and Optimization in Energy Systems)

Frontiers in Heat and Mass Transfer 2025, 23(6), 2109-2126. https://doi.org/10.32604/fhmt.2025.072643

Received 31 August 2025; Accepted 11 October 2025; Issue published 31 December 2025

Abstract

Internal thermal mass, such as furniture and partitions, plays a crucial role in enhancing building energy efficiency and indoor thermal comfort by passively regulating temperature fluctuations. However, the irregular geometry of these elements poses a significant challenge for accurate modeling in building energy simulations. This study addresses this gap by developing a rigorous analytical model that idealizes internal thermal mass as a sphere, thereby capturing multi-directional heat conduction effects that are neglected in simpler one-dimensional slab models. The transient heat conduction within the sphere is solved analytically using Duhamel’s theorem for three representative indoor air temperature scenarios: (1) constant, simulating a space with active HVAC; (2) exponentially decaying, representing a free-floating space after HVAC shutdown; and (3) periodically varying, corresponding to a naturally ventilated environment. Closed-form solutions are derived for the sphere’s internal temperature field, surface heat flux, and cumulative heat absorbed. The results demonstrate that a material’s Biot number governs its transient thermal response, with high-Biot-number materials (e.g., plywood) exhibiting a faster surface temperature rise but a steeper internal temperature gradient compared to low-Biot-number materials (e.g., concrete). The analysis of exponentially decaying and periodic scenarios reveals that sphere radius is the dominant factor determining total heat storage capacity; larger spheres absorb and release significantly more energy per cycle, despite having a lower heat flux density. Furthermore, a quantitative comparison of the decrement factor and time lag shows that while different materials may similarly dampen temperature amplitudes, a material with lower thermal diffusivity (like reinforced concrete) provides a substantially longer time lag, making it more effective for shifting thermal loads. This work provides a versatile and physically insightful analytical framework that advances the modeling accuracy of internal thermal mass beyond existing lumped-parameter methods.

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