BAB A - KONSEP-KONSEP ASAS MEKANIK BENDALIR



BAB 1 - FLUID AND PROPERTIES (BENDALIR DAN SIFATNYA)

Membincangkan tentang bendalir (gas atau cecair)

• Bendalir statik kaitan jisim, tenaga, daya, momentum dari sifat-sifat biasa bendalir

• Bendalir dinamik

- Fluid Mechanics is a section of applied mechanics, concerned with the static and dynamics of liquids and gases.

Fluid/Bendalir

Bendalir ≡ suatu bahan yang mengalami perubahan bentuk berterusan atau mengalir apabila dikenakan daya ricih. Ianya terdiri drp gas dan cecair.

• Bentuk bendalir berubah bila dikenakan daya, ia bergerak atau mengalir. Bentuknya sentiasa berubah selagi daya dikenakan

• Perubahan bentuk berlaku kerana daya ricih yang bertindak tangen kepada permukaan

PRESSURE (TEKANAN) :-

i. Atmosphere P. (Atmosfera), Po or Patm- ialah tekanan yang disebabkan oleh atmosfera dipermukaan bumi. Ia adalah bersamaan dengan 10.35m turus air atau 101.3kN/m2 atau 769mmHg (turus raksa) dari aras laut.

ii. Vaccum (Hampagas), Pv – ialah ruang yang kosong di mana tekanan adalah sifar.

iii. P.Gauge(Tolok), PG – ialah keamatan tekanan diukur berbanding dengan tekanan atmosfera.

iv. Absolute P.(Mutlak), PA – ialah keamatan tekanan diukur dari sifar mutlak, yakni hampagas sebenar.

Tekanan Mutlak = Tekanan Tolok + Tekanan Atmosfera.

1.Berapakah nilai tekanan Tolok, PG di dalam selinder udara, jika tekanan atmosfera, Po ialah 101.3 kN/m2 dan tekanan Mutlak, PA ialah 460 kN/m2.

pA = 460 kN/m2 patm = 101.3 kN/m2 pG = ?

Absolute pressure, PA = Gauge pressure, PG + atmospheric pressure, Po

pG = pA – pO

= 460 – 101.3

= 358.7kN/m2

Solve the problems below:

1. A Bourdon pressure gauge attached to a boiler located at sea level shows a reading pressure of 7 bar. If atmospheric pressure is 1.013 bar, what is the absolute pressure in that boiler (in kN/m2) ? 801.3kN/m2

PHYSICAL PROPERTIES OF FLUID (SIFAT-SIFAT FIZIKAL BENDALIR)

Untuk mengukur perbezaan kelakuan sifat-sifat bendalir tertentu digunakan. Sifat-sifat bendalir adalah ketumpatan jisim, berat tentu, graviti tentu, isipadu tentu dan kelikatan.

Sifat-sifat bendalir

1. Ketumpatan 2. Kelikatan 3. Tekanan wap

4. Kebolehmampatan 5. Tegangan permukaan 6. Kererambutan (capillarity)

Bahan, ρ (kgm-3)

Air 1000

Raksa 13.6

Udara 1.23 x 10-3

Paraffin oil 0.8

1.Mass density, ρ is defined as the mass per unit volume.( SI units, kg/m3 )

[pic]

2.Specific weight, ( is defined as the weight per unit volume.( SI units, N/m3 )

[pic],[pic],[pic] (where g = 9.81m/sec2)

In SI units the specific weight of water is 9.81 x 1000 = 9810 N/m3

3.Specific gravity or relative density, s is the ratio of the weight of the substance to the weight of an equal volume of water at 4 ºC.

[pic]or[pic]

4.Specific volume, v is defined as the reciprocal of mass density. It is used to mean volume per unit mass. (SI units, m3/kg ).

[pic] or [pic]

5.Viscosity - A fluid at rest cannot resist shearing forces but once it is in motion, shearing forces are set up between layers of fluid moving at different velocities. The viscosity of the fluid determines the ability of the fluid in resisting these shearing stresses.

Pekali kelikatan dinamik [pic]- daya ricih bg satu unit luas yg diperlukan utk menggerakkan satu lapisan bendalir.

Kelikatan Kinematik [pic]- nisbah diantara kelikatan dinamik dan ketumpatan jisim.

[pic]

1. What is the mass density (ketumpatan jisim), ρ of fluid (in kg/m3) if mass is 450 g and the volume is 9 cm3.

2. What is the specific volume, v of fluid

Solution

1. [pic] [pic] [pic]

2. [pic] [pic] [pic]

3. What is the specific weight, ( of fluid (in kN/m3) if the weight of fluid is 10N and the volume is 500 cm2.

[pic] [pic] = 20 000 N/m3 [pic]

4. What is the specific gravity of fluid in Example 1.2

[pic] [pic] [pic]

ASSESSMENT TASK

Q1.Januari 2004

a). Define the physical properties of the following;

i. Mass density

ii. Specific gravity

iii. Specific weight

iv. Specific volume (10 markah)

b). Tekanan tolok bendalir di dalam sebuah silinder ialah 350 kN/m2.

i. Cari tekanan di dalam ungkapan turus air dan turus raksa

ii. Tentukan tekanan mutlak di dalam silinder jika tekanan atmosfera ialah 101.3 kN/m2. (10 markah)

Q2. Julai 2004

a). If the mass and the respective volume of air is 5.11 kg and 650 cm3, calculate:

i. Mass density

ii. Specific gravity of air

iii. Specific weight

iv. Specific volume (14markah)

c). Define three (3) of the following pressure measurements:-

i. Atmospheric Pressure

ii. Pressure Gauge

iii. Absolute Pressure (6 markah)

Q3. Januari 2005

a). State definitions for the following;

i. pressure gauge

ii. absolute pressure

iii. atmospheric pressure

iv. vacuum pressure (10markah)

b). If the volume, mass and the dynamic viscosity of oil are 7.3 m3, 6500 kg and 10-3 N s/m2 respectively. Determine :

i. mass density

ii. Specific gravity

iii. specific weight

iv. Kinematic viscosity, v (10markah)

ListeRead phonetically

Q4. Julai 2005

a). Define the gauge pressure, atmospheric pressure and absolute pressure. (4markah)

b). Tinggi turus sejenis gas yang diambil dari sebuah tolok tekanan ialah 68mm air, manakala tinggi turus raksa akibat tekanan atmosfera ialah 750mm raksa. Berapakah nilai tekanan mutlak gas tersebut dalam unit kN/m2. (6markah)

c). Berat isipadu 1 liter air suatu bendalir adalah 7.05N. Berdasarkan kepada data-data tersebut, hitungkan ;

i. Ketumpatan jisim, [pic]

ii. Berat tentu, [pic]

iii. Ketumpatan bandingan, S.G

iv. Isipadu tentu, [pic] (10markah)

v.

Q5.

a) If 5.6 m3 of oil and weighs 46000N, determine :

a) Mass density

b) Specific weight

c) Specific gravity of oil (10 markah)

b) Calculate the specific weight of fluid in kN/m3 if the weight is 10N and volume of fluid 500 cm3 and determine mass density of fluid? (6markah)

c). Determine the specific volume of steam, if given the mass of steam 500g and the volume is 400 cm3. (4markah)

Q6.

a) Given specific weight of fluid is 6.54 kN/m3 and its mass is 8.3 kg, calculate the following:volume of fluid

a) specific volume of fluid

b) density of fluid (10 markah)

b) Given oil specific gravity is 0.89, find :

a) density of oil

b) specific weight of oil

c) specific volume of oil (10 markah)

M

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