Thermodynamic performance evaluation of an indoor designed solar air heater duct equipped with V-down baffle blocks having racetrack-shaped staggered openings of varying aspect ratio – an experimental study

ABSTRACT

1This paper presents the thermodynamic performance assessment of an indoor-designed solar air heater (SAH) duct with its absorber affixing the racetrack perforated 45^o V-down baffle blocks in staggered holes array. The experimentation work encompasses studying the effect of parameters like – the mass flow rate of air ($\dot{\mathrm{m}}$) and the hole aspect ratio (w/d) of baffle blocks on the air temperature rise (${\mathrm{\Delta }\mathrm{T}}_{\mathrm{r}\mathrm{i}\mathrm{s}\mathrm{e}}$), the energy (${\eta }_{\mathrm{I}}$), the exergy (${\eta }_{\mathrm{I}\mathrm{I}}$) and the thermohydraulic (${\eta }_{\mathrm{e}\mathrm{f}\mathrm{f}}$) efficiencies of the duct under turbulent airflow conditions. The open area ratio (Φ), relative roughness height (e/H) and relative roughness pitch (p/e) of the baffle blocks are maintained constant in this study. The result showed drastic improvement in the performance assessing parameters due to ‘w/d‘ affecting the heat transfer and friction characteristics significantly in the duct. Increasing ‘w/d’ at lower airflow rate results in a uniform downfall of ‘${\left(\mathrm{\Delta }\mathrm{T}\right)}_{\mathrm{r}\mathrm{i}\mathrm{s}\mathrm{e}}$’, and ‘${\eta }_{\mathrm{e}\mathrm{f}\mathrm{f}}$’. At an increased flow rate, the energy and exergy efficiencies decrease. The exergy destruction and pressure drop show non-monotonous variations with the hole-aspect ratio. The smallest hole aspect ratio (w/d of 1.04) baffle blocks outperform in all assessment segments providing maximum values as 14.86^oC, 96.81%, 17.46% and 93.95%, respectively, for the air temperature rise, the energy, the exergy and the thermohydraulic efficiency.

KEYWORDS:

• V-Baffle block
• staggered racetrack-shaped perforations
• hole aspect ratio
• energy efficiency
• exergy efficiency

Nomenclature

${\mathrm{A}}_{\mathrm{c}}$	=	cross section area of duct, m2
${\mathrm{A}}_{\mathrm{p}}$	=	surface area of absorber plate, m2
${\mathrm{A}}_{\mathrm{G}}$	=	gross area of collector/area of heating element, m2
${\mathrm{a}}_{\mathrm{o}}$	=	area of orifice, m2
${\mathrm{c}}_{\mathrm{p}\mathrm{a}}$	=	specific heat of air at constant pressure, J/kg K
${\mathrm{D}}_{\mathrm{h}}$	=	hydraulic diameter of duct, m
$\mathrm{D}$	=	diameter of circular G.I. pipe, m
$\mathrm{d}$	=	depth of racetrack hole, mm
${\mathrm{d}}_{\mathrm{h}}$	=	hydraulic diameter of racetrack hole, m
${\mathrm{d}}_{\mathrm{o}}$	=	diameter of orifice, m
$\mathrm{e}$	=	height of V baffle block, mm
$\mathrm{H}$	=	height of duct, m
$\mathrm{h}$	=	convective heat transfer coefficient, W/m2 K
$\mathrm{I}$	=	heat flux, W/m2
${\mathrm{k}}_{\mathrm{f}}$	=	thermal conductivity of fluid (air), W/m K
$\mathrm{L}$	=	length of test section of duct, m
$\dot{\mathrm{m}}$	=	mass flow rate of air, kg/s
$\mathrm{p}$	=	pitch of V baffle block, mm
${\mathrm{\Delta }\mathrm{P}}_{\mathrm{d}}$	=	pressure drop across duct test section, Pa.
${\mathrm{\Delta }\mathrm{P}}_{\mathrm{o}}$	=	pressure drop across orifice meter, Pa.
$\mathrm{P}\mathrm{m}$	=	pumping power, W
${\dot{\mathrm{Q}}}_{\mathrm{u}}$	=	useful heat gained by air, W.
${\dot{\mathrm{Q}}}_{\mathrm{c}}$	=	energy input to collector, W.
${\mathrm{T}}_{\mathrm{a}\mathrm{i}}$	=	temperature of air at inlet to duct, K
${\mathrm{T}}_{\mathrm{a}\mathrm{o}}$	=	temperature of air at exit from duct, K
${\mathrm{T}}_{\mathrm{b}\mathrm{m}}$	=	Bulk mean temperature of air, oC
${\mathrm{T}}_{\mathrm{p}}$	=	average temperature of absorber plate, K
$\mathrm{W}$	=	width of duct, m
$\mathrm{w}$	=	total width of racetrack hole, mm
Ẋ	=	Exergy, W
${\mathrm{I}}_{\mathrm{r}}$	=	Irreversibility, W

Greek symbols
$\alpha$	=	air angle of attack, degree
$\beta$	=	diameter ratio (= do/D), dimensionless
${\rho }_{\mathrm{a}}$	=	density of air, kg/m3
$\mu$	=	viscosity of air, kg/m-s
$\eta$	=	efficiency, dimensionless
$\psi$	=	specific exergy, J/kg

Subscripts
a	=	air
av	=	average
bm	=	bulk mean
d	=	duct
dest	=	destroyed
eff	=	effective/thermohydraulic
I	=	1st law
II	=	2nd law
p	=	pipe
s	=	smooth

Acknowledgement

The author acknowledges sincere thanks to the DIT University, Dehradun (Uttarakhand), for the financial support and providing the facility in conducting this research work.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.