版權(quán)說明:本文檔由用戶提供并上傳,收益歸屬內(nèi)容提供方,若內(nèi)容存在侵權(quán),請進行舉報或認領(lǐng)
文檔簡介
1、10.1 A droplet of water is falling through 20 0C air at the terminal velocity. The relative humidity of the air is 10% and the droplet is at the wet bulb temperature. The diffusivity of water vapor in air is 0.210-4 m2/s. Estimate the convective mass transfer coefficient for a 100 m diameter droplet
2、.Given: the dry bulb temperature Ta= 20 , =10%; the droplet is at the wet bulb temperature;The diffusivity of water vapor in air D= 0.2 10-4 m 2 /s. See Example 10.4Approach: Estimate the convective mass transfer coefficient for a 100 m diameter droplet.Solution:For mass transfer from a freely falli
3、ng liquid droplet, the following expression is recommended.1. The Reynolds number,T=20 , p=998.2 kg/m3, f=1.164 kg/m3; for air, =18.24010-6 Pa s;ut=0.298 m/sthe dry bulb temperature Ta= 20 , =10%, from Figure A.5.2, the droplet is at the wet bulb temperature Tw=7.6, Viscosity of water: 150 10-5 Pa.s
4、estimated;At temperature 7.6, for water, =999.8kg/m3;NRe=19.862. The Schmidt number isNSc=7.5010-23. The Sherwood number can be obtained from Equation (10.38)NSh=3.1284. The mass transfer coefficientkm=0.626 m/s10.2 The Sherwood Number for vapor transport from the surface of a high moisture food pro
5、duct to the surrounding air is 2.78.Compute the convective mass transfer coefficient, when the dimension of the product in the direction of air movement is 15 cm, and the mass diffusivity for water vapor in air is 1.810 -5 m 2/s.Given: The Sherwood Number for vapor transport from the surface of a hi
6、gh moisture food product to the surrounding air Nsh= 2.78.Approach: Compute the convective mass transfer coefficient, when the dimension of the product in the direction of air movement dc=15 cm=0.15m;the mass diffusivity for water vapor in air D= 1.8 10-5 m 2 /s.Solution:Using the formula 10.16 for
7、calculationkm=3.33610-4 m/s10.3 A flavor compound is held within a 5 mm diameter time release sphere. The sphere is placed in a liquid food and will release the flavor compound after one month of storage at 20 0C. The concentration of flavor within the sphere is 100% and the mass diffusivity for the
8、 flavor compound within the liquid food is 7.810-9 m2/s. Estimate the steady-state mass flux of flavor into the liquid food from the surface of the time-release sphere. The convective mass transfer coefficient is 50 m/s.Given: 5 mm diameter time release sphere, the storage temperature T=20;The conce
9、ntration of flavor within the sphere is 100% and the mass diffusivity for the flavor compound within the liquid food D=7.8 10-9 m2 /s.Approach: Estimate the steady-state mass flux of flavor into the liquid food from the surface of the time-release sphere. The convective mass transfer coefficient km
10、=50 m/s.Solution:1. For steady-state diffusion of gases (and Liquids) through solids;Assuming the mass diffusivity does not depend on concentration, from Equation (10.1) we obtainwhere DAB is mass diffusivity for gas A (or liquid A) in a solid B.Subscript A for m and c represents a gas or liquid dif
11、fusing through a solid. In reality, DAB represents an effective diffusivity through solids., At T=20, the saturated water vapor density =0.01834 kg/m3, 0, CA20;mA=4r1DABCA1=43.142.510-37.810-90.01834=4.4910-12 kg/s2.The convective mass transfer coefficient k m is defined as the rate of mass transfer
12、 per unit area per unit concentration difference. Thus,w here m B is the mass flux (kg/s); c is concentration of component B, mass per unit volume (kg/m 3); A is area (m 2). The units of k m are m 3/m2 s or m/s. The coefficient represents the volume (m3) of component B transported across a boundary
13、of one square meter per second; the surface area of the sphere A=d2=(510-3)2=7.8510-5 m2;mB=kmA(CB1-CB2)=507.8510-50.01834=7.2210-5 kg/s10.4 A desiccant is being used to remove water vapor from a stream of air with 90% RH at 50 0C. The desiccant is a flat plate with 25 cm length and 10 cm width, and
14、 maintains a water activity of 0.04 at the surface exposed to the air stream. The stream of air has velocity of 5 m/s moving over the surface in the direction of the 25 cm length. The mass diffusivity for water vapor in air is 0.1810-3 m 2/s. Compute the mass flux of water vapor from the air to the
15、desiccant surface.Given: the dry bulb temperature Ta= 50 , =90%; The desiccant flat plate with 25 cm length and 10 cm width, water activity aw=0.04;The air moving velocity u=5 m/s, the mass diffusivity for the flavor compound within the liquid food D=0.18 10-3 m2 /s.Approach: Compute the mass flux o
16、f water vapor from the air to the desiccant surface.We will first determine the Reynolds number and then use an appropriate dimensionless correlation to obtain the mass transfer coefficient and the water evaporation rate.see example 10.1Solution:1. u=5 m/s, from Table A.4.4, T=50 ,viscosity of air:
17、19.515 10-6 Pa s, =1.057kg/m3;NRe=6.77104NRe 100From Figure 4.40,=0.42.Thenf=1.067105s=29.63h3. Using Figure 4.42 at NBi 100,jm =0.74.Using the diffusion rate equation:log(0.2-0.15)=-t=17.45h10.6 An apple slice with 1 cm thickness is exposed to 80% RH for one week. After one week, the water activity
18、 of the apple has increased to 0.6, from an initial value of 0.1. The convective mass transfer coefficient at the product surface is 810-3 m/s. Estimate the mass diffusivity for water vapor in the apple.Given: An apple slice with 1 cm thickness; exposed to =80% for one week;After one week, water activity from aw=0.1 to aw=0.6;The convective mass transfer coefficient at the product surface km =810-3 m /s;see Example 10.7.Approach: Estimate the mass diffusiv
溫馨提示
- 1. 本站所有資源如無特殊說明,都需要本地電腦安裝OFFICE2007和PDF閱讀器。圖紙軟件為CAD,CAXA,PROE,UG,SolidWorks等.壓縮文件請下載最新的WinRAR軟件解壓。
- 2. 本站的文檔不包含任何第三方提供的附件圖紙等,如果需要附件,請聯(lián)系上傳者。文件的所有權(quán)益歸上傳用戶所有。
- 3. 本站RAR壓縮包中若帶圖紙,網(wǎng)頁內(nèi)容里面會有圖紙預(yù)覽,若沒有圖紙預(yù)覽就沒有圖紙。
- 4. 未經(jīng)權(quán)益所有人同意不得將文件中的內(nèi)容挪作商業(yè)或盈利用途。
- 5. 人人文庫網(wǎng)僅提供信息存儲空間,僅對用戶上傳內(nèi)容的表現(xiàn)方式做保護處理,對用戶上傳分享的文檔內(nèi)容本身不做任何修改或編輯,并不能對任何下載內(nèi)容負責(zé)。
- 6. 下載文件中如有侵權(quán)或不適當(dāng)內(nèi)容,請與我們聯(lián)系,我們立即糾正。
- 7. 本站不保證下載資源的準(zhǔn)確性、安全性和完整性, 同時也不承擔(dān)用戶因使用這些下載資源對自己和他人造成任何形式的傷害或損失。
最新文檔
- 2024年車輛內(nèi)裝產(chǎn)品項目發(fā)展計劃
- 2024年醫(yī)用供應(yīng)裝置合作協(xié)議書
- 2024年涂鍍中間體項目發(fā)展計劃
- 2024年彈射救生系統(tǒng)項目合作計劃書
- 2024年沙盤模型制作項目合作計劃書
- 2024年煤制天然氣項目建議書
- 2024版分期付款買賣合同示范文本
- 2024版合同工程驗收鑒定書
- 飲品店陽臺整修合同
- 2024版燈光設(shè)備租賃合同
- 2023全國《行政單位會計制度》知識競賽題庫及答案
- 客運站安全生產(chǎn)風(fēng)險分級管控和隱患排查治理雙體系方案全套資料(2021-2022版)
- GB/T 2085.2-2007鋁粉第2部分:球磨鋁粉
- 信息技術(shù)課評價方案范文
- 臨床常見問題的康復(fù)評定與處理
- Unit3 Topic2-SectionA課件- 仁愛版九年級英語上冊
- 新蘇教版六上科學(xué)6.《生物的變異》課件
- ASTM-E505 標(biāo)準(zhǔn)圖譜參考資料 標(biāo)準(zhǔn)講義
- 三年級上冊語文課件-《24司馬光》(共15張PPT) 人教部編版
- 尺骨冠狀突骨折護理查房課件
- 藥物代謝酶基因檢測指導(dǎo)個體化用藥課件
評論
0/150
提交評論