全球氣候變遷促進節能減碳與綠能研究之蓬勃發展，然而各類綠能，如太陽能、風能、海洋能及地熱等可持續能源，大部分均轉換成電能，馬達為轉換電能至機械能之旋轉機械，因此發展高功率密度之馬達，將各式綠能轉換成電能運用之重要性亦隨之提高。本研究針對發展高功率馬達所需之散熱技術，分別新創馬達轉軸、轉子、定子之高效能冷卻技術。
由於馬達動力輸出端通常需要與機械負載聯結，因此發展由轉軸單一側進出冷卻流體之水冷技術，應用衝擊噴柱陣列以及環流道設置紊流產生器，提升旋轉中空軸內部之冷卻效應。研究重點探討旋轉對於衝擊噴柱陣列以及三類環流道之熱傳性能、壓損增益所產生之影響。研究結果顯示，在科氏力與橫流效應之共同作用下，噴柱於旋轉中空軸中被科式力扭曲，且橫流效應同時稀釋噴柱動量，削弱衝擊噴柱區域之冷卻效能。衝擊噴柱流域下游之環流域，出現緊密聯結之渦核結構(coherent vortex cores)，當旋轉雷諾數(Reω)從零開始增加時，此流域之平均紐賽數比值最初自1開始上升，隨後進一步增加Reω後，熱傳增益轉為隨Reω增加而下降。受到衝擊噴柱流域之噴柱扭曲及動量稀釋導致的強化剪切作用，與環流域中之渦流結構變化，旋轉與靜止條件之摩擦係數比(f/f0)隨Reω增加而上升。應用本研究開發的三維馬達熱場計算方法，並評估定泵功率條件三種軸冷系統，分別為平滑環流(SW)、柱鰭環流(PF)、螺旋肋環流(SR)之空心轉軸內表面熱傳係數，計算使用SW、PF及SR轉軸冷卻裝置，分別提高旋轉狀態之轉子磁條冷卻效率至72.5-75.3%、69.73-75.57%及71.91-75.7%，基於提升Reω產生之旋轉表面紐賽數提高效應，以及於定轉矩條件導致之馬達元件熱功率增加，兩者因提高轉速對馬達元件最高溫(Tmax)產生之相互競爭影響，導致配置本研究開發之SW、PF及SR轉軸冷卻系統值，均隨轉速或Reω提升而下降。
轉子冷卻方面，本研究利用旋轉產生之離心力，開發旋轉環路熱虹吸管(RLT)，強化環路熱虹吸管中之汽液分離效果，提高相變化熱傳效益，降低熱虹吸管熱阻，增加等效導熱率，為新創之低耗能轉子冷卻系統。研究結果顯示， RLT中蒸發段紐賽數隨無因次熱通量(Q*)增加而降低，但隨離心力相對強度提高而上升，當提升Q*和/或無因次離心加速度(Ca數)時，RLT之壓力提升，伴隨冷凝段飽和溫度增高，同時冷凝段之汽液分離力亦隨Ca數增加而強化，致使所有冷凝段之紐賽數均隨Q*及Ca數增加而上升。而增加Ca數，提高RLT冷凝段外表面周圍氣流之速度，致使冷凝段外表面紐賽數隨Ca數增加呈冪律形式提升，將RLT充填率自0.5增加至0.8，蒸發段內側直管之液膜使加熱過程從過熱狀態轉變為飽和狀態，改善所有熱傳性能指標；於充填率為0.8的RLT內側直管中，螺旋線圈導引泵送效應，改善無螺旋線圈配置之RLT熱傳性能。基於Q*及Ca數對相變化熱傳產生之耦合作用，RLT之等效導熱率(Keff)依序為：配置螺旋線圈且FR=0.8之RLT > 未配置螺旋線圈且FR=0.8之RLT > 未配置螺旋線圈且FR=0.5之RLT > 配置螺旋線圈且FR=0.5之RLT。於本研究之測試條件，充填率為0.8且配置螺旋線圈之RLT，其等效熱導率達161.19-562.43 Wm-1K-1之範圍，提升至RLT管壁導熱率的107.5%-375%。
本研究開發扭轉式流道做為定子冷卻水套之熱傳強化方法，藉由冷卻流體(水)流經扭轉方形流道所導引出之二次流，與螺旋流道離心力產生之迪恩渦流協同作用，提高其漩流強度，提升流道之區域平均紐賽數。研究結果顯示，扭轉式流道之區域平均紐賽數隨流道節距降低而提升。於1921< De(迪恩數)<6455 和5000< Re<25000的條件，對於流道節距比為3、4、5和6的扭轉螺旋流道，其平均紐賽數與Dittus-Boelter紐賽數基準之比值分別提高至1.11-1.19、1.08-1.14、1.06-1.1和1.02-1.05。由於節距比為3之扭轉螺旋流道具有高熱傳強化效益，其熱性能係數(TPF)值高(優)於節距比為4、5和6之扭轉式流道。相較於未扭轉式流道，扭轉式流道展現出約18%之傳熱提升效益，同時保持與未扭轉式流道相近之TPF值，足證扭轉螺旋流道優異之熱傳性能。
本研究所開發之馬達轉軸、轉子、定子熱傳強化技術，可視馬達功率密度提升之需求，分別或合併使用，應用本研究之實驗與數值方法揭櫫之對流與熱傳現象，助益熱流領域之研究發展，基於其優異之熱傳強化性能，本研究新創之各式馬達冷卻技術，同時助益電機機械之發展，活化綠能應用。

The global climate change has urged the vigorous development of energy conservation, carbon reduction, and green energy research. However, most sustainable energy sources, such as solar, wind, ocean, and geothermal energy, are converted into electrical energy. Motors, as rotating machines that convert electrical energy into mechanical energy, have seen their importance rise in harnessing various green energy sources. Therefore, the development of high-power-density motors has become crucial. The present study focuses on the critical cooling technologies for developing high-power-density motors, proposing innovative and high-effective cooling measures for the shaft, rotor, and stator of an electric motor (EM). As the power output end of an EM is usually connected to a mechanical load, a water-cooling technology with single-end entry and exit of coolant flow was devised. This technology employs an impinging jet array and the turbulators installed in a downstream annular flow channel to enhance the cooling effect inside a rotating hollow shaft. Researches focus on investigating the influence of rotation on the thermal performance and pressure-drop penalty of the impinging jet array with three types of annular flow channels. Results show that under the combined effects of Coriolis force and crossflow, the jets in the rotating hollow shaft are distorted by the Coriolis force, while the crossflow effect simultaneously diffuse the jet momentum, hence weakening the cooling performance in the impingement zone. In the annular flow region downstream of the impinging jet flow, the interconnected coherent vortex cores are observed. When the rotational Reynolds number (Reω) increases from zero, the average Nusselt number ratio in this flow region initially rises from unity, but as Reω increases further, the heat transfer enhancement turns to decrease with increasing Reω. Due to the intensified shearing effects caused by jet distortion and momentum diffusion in the impinging jet region, as well as the modifications of vortex structure in the annular flow region, the friction factor ratio (f/f0) between those measured in rotating (f) and stationary (f0) conditions increases with Reω.
Using the three-dimensional motor thermal field simulation method developed by present research group, the heat transfer coefficients on the inner surface of the hollow rotating shaft were evaluated for three axial cooling schemes, namely smooth-walled (SW), pin-fined (PF), and spirally ribbed (SR) configurations in annular flow passage, under constant pumping powers. The SW, PF, and SR shaft cooling schemes improved the cooling efficiency are elevated by 72.5–75.3%, 69.73–75.57%, and 71.91–75.7% for the rotor magnetic strips under rotating conditions respectively. The enhancement in cooling efficiency is attributed to the increased rotational surface Nusselt number, resulting from higher Reω. Under constant torque conditions, the increased rotational speed also leads to higher thermal power generation in motor components. These two competing effects, one is the enhanced heat transfer rate due to higher Reω whist the other is the increased thermal power due to higher rotating speed, result in a η decrease for SW, PF, and SR shaft cooling schemes as rotational speed (or Reω) increases.
Regarding rotor cooling, present study utilizes the centrifugal force generated by rotation to develop a rotating loop thermosyphon (RLT), enhancing the vapor-liquid separation effect in the thermosyphon loop, improving phase-change heat transfer efficiency, reducing the thermal resistance of the thermosyphon, and increasing the equivalent thermal conductivity. This cooling concept permits an innovative low-energy-consumption rotor cooling solution. The research results indicate that in the RLT, the Nusselt number in the evaporator section decreases as the dimensionless heat flux (Q*) increases but increases with the relative intensity of centrifugal force. When Q* and/or the dimensionless centrifugal acceleration (Ca number) are increased, the pressure in the RLT rises, accompanying with an increase in saturation temperature over the condenser section. Simultaneously, the vapor-liquid separation force in the condenser section is also enhanced with an increase in the Ca, leading to an increase in the Nusselt number across all condenser sections as Q* and Ca increase. By increasing Ca, the velocity of the surrounding airflow on the outer surface of the RLT's condensation section was increased, resulting in a power-law increase of the Nusselt number over the outer surface of the condensation section with rising Ca. When the filling ratio (FR) of the RLT was increased from 0.5 to 0.8, the liquid film in the inner branch tubes of the evaporation section shifted the heating process from a superheated state to a saturated state, thereby improving all heat transfer performance indices. In the inner branch tubes of the RLT with FR=0.8, the pumping effect induced by the spiral coil improved the heat transfer performance compared to RLTs without spiral coils. Based on the combined effects of Q* and Ca on the phase-change heat transfer, the ranking of the RLT's effective thermal conductivity (Keff) followed the order of: RLT with spiral coil with FR=0.8 > RLT without spiral coil with FR=0.8 > RLT without spiral coil with FR=0.5 > RLT with spiral coil with FR=0.5. Under present test conditions, the RLT equipped with a spiral coil for a filling ratio of 0.8 achieved an effective thermal conductivity ranging from 161.19 to 562.43 W·m⁻¹·K⁻¹, representing an enhancement of 107.5% to 375% compared to the thermal conductivity of the RLT wall material.
Present study developed a twisted spiral channel as a heat transfer enhancement method for water jackets of stator cooling in an EM. By utilizing the secondary flow induced by the coolant (water) passing through the twisted square channel, triggering the synergistic effect of Dean vortices generated by centrifugal force and flow spirals induced by the twisted duct, the swirl intensity was enhanced, thereby improving the local average Nusselt number of the channel. The results showed that the local average Nusselt number of the twisted spiral channel increased as the twist pitch decreased. Under the conditions of 1921 < De (Dean number) < 6455 and 5000 < Re < 25000 for twisted spiral channels with pitch ratios of 3, 4, 5, and 6, the ratio of the average Nusselt number to the Dittus-Boelter Nusselt number reference was increased to 1.11–1.19, 1.08–1.14, 1.06–1.10, and 1.02–1.05, respectively. Since the twisted spiral channel with a twist pitch ratio of 3 exhibited high heat transfer enhancement, its thermal performance factor (TPF) was higher than those of the twisted spiral channels with twist pitch ratios of 4, 5, and 6. Compared to the non-twisted spiral channels, the twisted spiral channel demonstrated approximately an 18% improvement in heat transfer rate, while maintaining TPF values comparable to those of non-twisted spiral channels, confirming the superior hydrothermal performance for the twisted spiral channel.
The heat transfer enhancement technologies for shaft, rotor, and stator cooling, developed by present study, can be applied individually or in combination, depending on the power density requirements for an EM. By utilizing the experimental and numerical methods developed by present study, the revealed heat transfer phenomena contribute to advancements in the field of thermal-fluid research. Due to their outstanding performances in heat transfer enhancement, the innovative EM cooling technologies developed by present study also promote the development of electrical machinery that invigorate green energy utilizations.

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