Abstract
This study was conducted to examine the effect of relative roughness pitch (p/e) on
the characteristics of heat transfer and friction factor on absorber plate with transverse
continuous ribs artificial roughness in solar air heater duct. In this study, relative
roughness pitch with transverse continuous ribs (p/e) was varied at 8, 9, 10 and 11. Solar
air heater duct having the duct aspect ratio (W/H) was 12, and constant relative roughness
height (e/Dh) value at 0.033. To simulate the indoor testing of solar air heater, the absorber
plate was heated by an electric heater that provided constant heat flux of 1000 W/m2.
While the others sides of duct were insolated. Tests were performed on the air flow
Reynolds number (Re) of 3440 – 9990. The results of heat transfer and friction factor have
been compared with smooth plate under similar flow and thermal boundary condition to
determine the enhancement in heat transfer and friction factor. The results showed that the
Nusselt number and friction factor of the absorber plate with transverse continuous ribs
artificial roughness increases with the increase in the value of p/e until it reaches a
maximum at p/e = 10, after that experienced a decline. Nusselt number of the absorber
plate with transverse continuous ribs artificial roughness increase in the range of 32% -
47%, 44% - 64%, 57% - 92% and 35% - 55% compared to smooth plate for the values of
p/e = 8, 9, 10 and 11, respectively. Maximum enhancement of friction factor of the
absorber plate with transverse continuous ribs artificial roughness was obtained of 1.58,
1.74, 1.87 and 1.66 times friction factor of smooth plate for the values of p/e = 8, 9, 10 and
11, respectively. Maximum enhancement of Nusselt number and friction factor of the
absorber plate with transverse continuous ribs artificial roughness was obtained
respectively of 1.92 and 1.87 times compared to smooth plate at p/e = 10 and at Re = 9880
and Re = 3460, respectively. Thermo-hydraulic performance values of the absorber plate
with transverse continuous ribs artificial roughness in the range of 1.00 – 1.18, 1.02 – 1.21,
1.04 – 1.22 and 1.05 – 1.24 for the values of p/e = 8, 9, 10 and 11, respectively. Using
experimental data correlations for Nusselt number and friction factor have also been
developed for such solar air heater, which gives a good agreement between predicted
values and experimental values of Nusselt number and friction factor.
Keywords: absorber plate, artificial roughness, relative roughness pitch, solar air heater,
transverse continuous ribs.
Abstrak
Penelitian ini dilakukan untuk menguji pengaruh relative roughness pitch (p/e)
terhadap karakteristik perpindahan panas dan faktor gesekan pada plat penyerap dengan
kekasaran buatan transverse continuous ribs dalam saluran pemanas udara surya. Pada
penelitian ini, p/e divariasi sebesar 8, 9, 10 dan 11. Saluran pemanas udara surya
mempunyai duct aspect ratio (W/H) sebesar 12, dan relative roughness height (e/Dh)
konstan sebesar 0,033. Untuk mensimulasikan pengujian pemanas udara surya di dalam
ruangan, plat penyerap dipanaskan dengan pemanas listrik yang memberikan fluks kalor
konstan sebesar 1000 W/m2 dan sisi saluran lainnya diisolasi. Pengujian dilakukan pada
bilangan Reynolds aliran udara (Re) sebesar 3440 – 9990. Hasil perpindahan panas dan
faktor gesekan dibandingkan dengan plat penyerap halus (smooth plate) dibawah kondisi
aliran udara dan kondisi batas termal yang sama untuk menentukan peningkatan
perpindahan panas dan faktor gesekan. Hasil penelitian menunjukkan bahwa bilangan
Nusselt dan faktor gesekan dari plat penyerap dengan kekasaran buatan transverse
continuous ribs meningkat dengan kenaikan p/e hingga mencapai maksimum pada p/e =10,
setelah itu mengalami penurunan. Bilangan Nusselt plat penyerap dengan kekasaran buatan
transverse continuous ribs meningkat dalam kisaran 32% - 47%, 44% - 64%, 57% - 92%
dan 35% - 55% dibandingkan dengan smooth plate berturut-turut untuk nilai p/e=8, 9, 10
dan 11. Peningkatan faktor gesekan plat penyerap dengan kekasaran buatan transverse
continuous ribs maksimum didapatkan sebesar 1,58; 1,74; 1,87 dan 1,66 kali faktor
gesekan smooth plate berturut-turut untuk nilai p/e =8, 9, 10 dan 11. Peningkatan bilangan
Nusselt dan faktor gesekan maksimum dari plat penyerap dengan kekasaran buatan
transverse continuous ribs didapatkan berturut-turut 1,92 dan 1,87 kali dibandingkan
smooth plate pada p/e = 10 dan berturut-turut pada Re = 9880 dan Re = 3460. Nilai unjuk
kerja termohidrolik plat penyerap dengan kekasaran buatan transverse continuous ribs
dalam kisaran1,00 – 1,18; 1,02 – 1,21; 1,04 - 1,22 dan 1,05 – 1,24 berturut-turut untuk nilai
p/e =8, 9, 10 dan 11. Menggunakan data eksperimen, korelasi-korelasi untuk bilangan
Nusselt dan faktor gesekan juga dikembangkan untuk pemanas udara surya dan
memberikan kesesuaian yang baik antara nilai-nilai prediksi dan nilai-nilai eksperimen dari
bilangan Nusselt dan faktor gesekan.
Kata kunci: kekasaran buatan, pemanas udara surya, plat penyerap, relative roughness
pitch, transverse continuous ribs.