工程熱力學(xué)與傳熱學(xué)chapter7MoistairAtmospherica.ppt

,Chapter 7 Moist air /Atmospheric air,7-1. State property of moist air,7-4. Thermodynamics process of moist air,內(nèi)容提要,7-2. h-d diagrams of moist air,7-3. Dew-point temperature and Wet-bulb temperature,1.Dry and moist air,7-1 State property of moist air,Air in the atmosphere normally contains some water vapor and is referred to as moist air. By contrast, air that contains no water vapor is called dry air.,It is often convenient to treat air as a mixture of water vapor and dry air since the composition of dry air remains relatively constant.,moist air = dry air + water vapor,道爾頓分壓定律,飽和蒸汽,1、unsaturated air,未飽和濕空氣和飽和濕空氣,過熱蒸汽,水蒸汽,干空氣過熱水蒸汽,pw ps(T),7-1 State property of moist air,unsaturated air Saturated air?,2、 Saturated air,未飽和濕空氣和飽和濕空氣,pw ps(T),T,s,ps,溫度一定，不能再加入水蒸汽,從未飽和到飽和的途徑,T,s,pw,ps,a,b,d,c,e,結(jié)露,Td 露點(diǎn)溫度,一定溫度條件下，濕空氣所含水蒸汽的分壓力的極限是飽和分壓力。濕空氣所含水蒸汽的分壓力大小是衡量濕空氣干燥與潮濕程度的基本指標(biāo)。,Td,濕空氣中所含水蒸汽的量,濕空氣的濕度,1、絕對濕度,3、含濕量,d,2、相對濕度,濕空氣的濕度,1、絕對濕度,1m3濕空氣中所含水蒸汽的質(zhì)量為濕空氣的絕對濕度。,絕對濕度只能說明濕空氣中實(shí)際所含水蒸汽的多少，不能說明濕空氣所具有的吸收水蒸汽的能力大小。,7-1 State property of moist air,3.specific and relative humidity, 1 saturated air, 0 dry air,(0-1) unsaturated air,(2)relative humidity,(1)specific/Absolute humidity,2、相對濕度, 1 飽和濕空氣,在相同的溫度下：,相對濕度：濕空氣中水蒸汽的實(shí)際含量與同溫度下最大可能含量的比值。, 0 干空氣,0 1 未飽和濕空氣,表明濕空氣與同溫下飽和濕空氣的偏離程度,反映所含水蒸汽的飽和程度,3、 含濕量,濕空氣中干空氣的量總不變，以此為計(jì)算基準(zhǔn),含濕量：含有1kg干空氣的濕空氣中所攜帶的水蒸汽質(zhì)量。,g水蒸氣/kg干空氣,B為定值時(shí)，空氣中水蒸汽的含量只取決于濕空氣中水蒸汽分壓力pw的大小，d=(pw).含濕量是隨水蒸汽分壓力的大小而增減的。,干球溫度和濕球溫度,兩只相同的溫度計(jì)，一支的感濕包用侵于水中的濕紗布包起來，稱為濕球溫度計(jì)。,濕空氣處于飽和狀態(tài)時(shí)，濕紗布上的水分不蒸發(fā)，干球溫度=濕球溫度。,濕空氣未飽和時(shí)，濕紗布上的水分會(huì)蒸發(fā)，水溫下降，濕球與周圍的空氣存在溫差，空氣向濕紗布傳熱，當(dāng)這些熱量不足以補(bǔ)償水分蒸發(fā)所消耗熱量時(shí)，要從紗布的水分吸熱，使?jié)袂虻乃疁乩^續(xù)下降，當(dāng)溫度降到一定程度，外界傳入紗布的熱量等于水分蒸發(fā)需要的熱量時(shí)，溫度保持不變，這就是濕球溫度tw。,干球溫度和濕球溫度,空氣的相對濕度越小，濕球溫度比干球溫度就低得越多。,對于未飽和濕空氣，濕球溫度總是介于露點(diǎn)溫度和干球溫度之間：tdtwt。,飽和濕空氣，這三種溫度相等： td=tw=t,td = ts(pw),結(jié)露與露點(diǎn),濕潤的夏天水管上常出現(xiàn)水珠?,冷水管t=20oC,T,s,d,td=28.98oC,未飽和濕空氣在水蒸汽分壓力不變的情況下，冷卻至飽和濕空氣時(shí)的溫度稱為露點(diǎn)，td。,霧、云及霜怎么形成的？,T,s,d,在水氣充足、微風(fēng)及大氣層穩(wěn)定的情況下，如果接近地面的空氣冷卻至某程度時(shí)，空氣中的水氣便會(huì)凝結(jié)成細(xì)微的水滴懸浮于空中，使地面水平的能見度下降，這種天氣現(xiàn)象稱為霧。,如果有大量的凝結(jié)核存在，更容易生成霧。一般在工業(yè)區(qū)和城市中心形成霧的機(jī)會(huì)更多，因?yàn)槟抢镉胸S富的凝結(jié)核存在。,td0.0 oC,td 0.0 oC時(shí)，就形成霜。,7-1 State property of moist air,3.specific and relative humidity, 1 saturated air, 0 dry air,(0-1) unsaturated air,(2)relative humidity,(1)specific/Absolute humidity,4.The total enthalpy of moist air,7-1 State property of moist air,7-2 h-d diagrams of moist air,1. h, d, T, lines,Software!,7-2 h-d diagrams of moist air,2.熱濕比,7-2 h-d diagrams of moist air,熱濕比:,7-3 Dew-point and Wet-bulb temperature,1.Dew-point,The dew-point temperature is defined as the tempera-ture at which condensation begins when the air is cooled at constant pressure.,unsaturated air,Dew-point,unsaturated air,Dew-point,濕球溫度無法直接測量,只能采用間接方法測量.,2,2.Wet-bulb temperature (濕球溫度),如上圖:使未飽和的濕空氣1與水有足夠的接觸表面和時(shí)間.則出來2點(diǎn)的將是飽和的濕空氣,2點(diǎn)的溫度稱為濕球溫度.它比濕空氣本身的溫度(干球溫度)要低.,7-3 Dew-point and Wet-bulb temperature,Tw,T,如圖:干球溫度計(jì)的感溫包上包一塊濕布,濕布放于水缸中,就是濕球溫度計(jì),測得的溫度可近似認(rèn)為是濕球溫度.,由于水蒸發(fā)都要吸收空氣的熱量,所以: 濕球溫度Tw干球溫度T 并且