| Abstract: | 本研究實驗探討氣流以不同斜向角度進入管道時，管道兩熱傳面上熱傳及流場特
性。一小型風洞系統用來產生均勻流於一3.5公分高*7公分寬管道入口，此管道
包括5公分長之前置直管道導引氣流，以不同角度，0度、15度、30度、45度、
60度、75度，連接25 公分長之測試直管道；測試管道上下壁面上龕置有不銹鋼
薄片，作為熱傳面；流場雷諾數介於2350及23500間。量測內容包括以熱電偶作
兩熱傳面上溫度分佈量測，以獲得Nusselt Number分佈；以雷射測速儀作管道斷
面及近熱傳表面穩態及非穩態速度量測，以探討流場特性如二次流強度、軸向速
度及紊流動能與熱傳關聯。研究結果顯示，雖然管道角度愈大有較大之壓力損失
，但仍有較佳之熱傳效果，氣流流經衝擊面(上熱傳面)有較大之軸向速度
(convective effect)，而在非衝擊面(下熱傳面)附近雖有迴流區產生，但有較
大紊流動能；流體流經衝擊面及非衝擊面上之二次流強度與熱傳之Nusselt
number分佈接近，顯示在衝擊面及非衝擊面上之熱傳特性由二次流強度所主導。
一般而言，除了在管道入口附近，非衝擊面上之熱傳效果較衝擊面之熱傳效果為
佳。
The research experimentally investigates the heat transfer and flow
characteristics on impinging and non-impinging heat transfer surfaces
in oblique duct flows. A small wind tunnel system is used to generate
a uniform flow at the inlet of a 3.5*7 cm/sup 2/ duct that includes a
5 cm duct connected to a 25 cm test duct with varied angles of 0/sup
.degree./, 15/sup .degree./, 30/sup .degree./, 45/sup .degree./,
60/sup .degree./ and 75/sup .degree./. Two stainless steel foils are
attached to the duct top- and bottom-wall to serve as heat transfer
surfaces. The flow Reynolds numbers are between 2350 and 23500. The
studies include temperature measurements on the heat transfer surfaces
by thermocouples to obtain the Nusselt number distributions.
Three-component mean and fluctuating velocity measurements were
conducted at duct cross sections and near the heat transfer surfaces,
using a laser Doppler velocimetry, to characterize the flows. Results
of this study indicate that the flows with larger angles into the duct
cause larger pressure drops, as expected, but having larger heat
transfer rates. The flows over the impinging heat transfer surface
have larger axial mean velocity (convective effect), while the flows
over the non-impinging heat transfer surface have larger turbulent
kinetic energy (turbulent effect). The Nusselt number distributions on
the impinging and non-impinging heat transfer surfaces are similar to
the distributions of the secondary-flow strength, indicating the
secondary flow is the dominating effect on the heat transfer rate. In
general, the heat transfer performance on the non-impinging surface is
better than that on the impinging surface except in the region near
the duct inlet.
The research experimentally investigates the heat transfer and flow characteristics on impinging and non-impinging heat transfer surfaces in oblique duct flows. A small wind tunnel system is used to generate a uniform flow at the inlet of a 3.5*7 cm/sup 2/ duct that includes a 5 cm duct connected to a 25 cm test duct with varied angles of 0/sup .degree./, 15/sup .degree./, 30/sup .degree./, 45/sup .degree./, 60/sup .degree./ and 75/sup .degree./. Two stainless steel foils are attached to the duct top- and bottom-wall to serve as heat transfer surfaces. The flow Reynolds numbers are between 2350 and 23500. The studies include temperature measurements on the heat transfer surfaces by thermocouples to obtain the Nusselt number distributions. Three-component mean and fluctuating velocity measurements were conducted at duct cross sections and near the heat transfer surfaces, using a laser Doppler velocimetry, to characterize the flows. Results of this study indicate that the flows with larger angles into the duct cause larger pressure drops, as expected, but having larger heat transfer rates. The flows over the impinging heat transfer surface have larger axial mean velocity (convective effect), while the flows over the non-impinging heat transfer surface have larger turbulent kinetic energy (turbulent effect). The Nusselt number distributions on the impinging and non-impinging heat transfer surfaces are similar to the distributions of the secondary-flow strength, indicating the secondary flow is the dominating effect on the heat transfer rate. In general, the heat transfer performance on the non-impinging surface is better than that on the impinging surface except in the region near the duct inlet. |
