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Research Papers

# A Transient Immersed Coil Heat Exchanger Model

[+] Author and Article Information
William R. Logie

e-mail: william.logie@solarenergy.ch

Elimar Frank

Institut für Solartechnik SPF,
HSR Hochschule für Technik Rapperswil,
Oberseestrasse 10, 8640, Rapperswil, Switzerland

A side effect of this higher temperature is a drop in the collector efficiency.

All reference to low flow refers to the flow conditions of the collector and not the immersed heat exchanger.

While all Pt $100 Ω$ sensors were regularly calibrated, the error implied here is that of resolution, seeing as a maximum of 12 sensors were used to collect local wall temperature information.

Neumann boundary conditions with surface heat flux set to zero.

In the TRNSYS environment it is typical to evaluate an average power over the time step, in which case the average is used for both the old and new power densities.

Contributed by the Solar Energy Division of ASME for publication in the Journal of Solar Energy Engineering. Manuscript received September 12, 2012; final manuscript received February 14, 2013; published online June 25, 2013. Assoc. Editor: Werner Platzer.

J. Sol. Energy Eng 135(4), 041006 (Jun 25, 2013) (7 pages) Paper No: SOL-12-1225; doi: 10.1115/1.4023928 History: Received September 12, 2012; Revised February 14, 2013

## Abstract

The aim of this paper is to present a transient one-dimensional (1D) radial immersed coil heat exchanger model that accounts for the effect that geometry and operating conditions have on heat transfer performance. Insights gained through its use in both an analysis of experimental data and an implementation in the simulation environment TRNSYS are shown and discussed. While variation in the external convection coefficient of immersed coil heat exchangers has little effect on the annual solar fraction of a generic solar domestic hot water system, variation in collector side flow can influence the solar fraction as great as ±5%, in particular low collector side flow improves stratification inside the store.

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## References

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## Figures

Fig. 1

A temperature array used for modeling stratification in thermal energy stores

Fig. 2

Illustration of a TES with an IHX showing an exploded view of a discrete IHX section with tube length ∂L and surface area ∂A

Fig. 3

Varied flow (Reynolds number) overall heat exchange characteristics of a stainless steel tube coiled through a diameter of 0.49 m to a length of 20.45 m and an area of 1.69 m2; internal fluid is 33% ethylene-glycol, storage fluid is water parametrically set at a constant 30 °C and a collector power of 2 kW is assumed to be leaving the heat exchanger

Fig. 4

Plots of local heat transfer information over the height of a discretized exemplary IHX helix (dimensions and operating conditions as in Fig. 3)

Fig. 5

Overall heat transfer area coefficient of seven immersed heat exchangers tested over three mass flow rates with a constant power of 2 kW

Fig. 6

Plot of external Nusselt numbers against the respective Rayleigh numbers. The correlation from Morgan [34] and two variations either side of this are also plotted for comparison.

Fig. 7

Illustration of Crank–Nicolson discretization

Fig. 8

Illustration of horizontal tube cross section showing development of the convection thermal boundary layer given the presence of heat exchange from the internal to the external fluid

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