JSFiddle - React, Tailwind, and code Playground

JavaScript

V=25/3600  // flow rate, m³/s
d=0.1  // diameter, meters
t1=-30 // fluid inlet temperature
t2=4.9 // fluid  outlet temperature
ts=5  // tube surface temperature

tf=(t1+t2)/2  // fluid temperature
document.write(`tf = ${tf} °C<br>`)

// values from http://www.mhtl.uwaterloo.ca/old/onlinetools/airprop/airprop.html :
nu=1.2246*10**-5 // kinematic viscosity
Prf=0.72275  // prandtl number for fluid temperature
Prs=0.71866  // prandtl number for tube surface temperature
la=0.023096  // thermal conductivity
rho=1.3558  // density
Cp=1.0054*10**3  // specific heat capacity, J/kg*K

f=Math.PI*d**2/4  // cross-sectional area
w=V/f  // velocity of the fluid, m/s
Re=w*d/nu  // reynolds number
if (Re <= 2300) {
	document.write('Re <= 2300<br>')
} else {
	document.write(`Re = ${Re}<br>`)
	el=1  // for L/d >= 50 el=1
	ep=0.93  // from table 3.3
	Nus=0.021*Re**0.8*Prf**0.43*(Prf/Prs)**0.25*el*ep  // nusselt number
	al=Nus*la/d  // heat transfer coefficient
	G=V*rho  // mass flow rate, kg/s
	Q=G*Cp*(t2-t1)  // heat flow, W
	lmtd=(t2-t1)/Math.log((ts-t1)/(ts-t2))  // logarithmic mean temperature difference
	l=Q/(al*lmtd*Math.PI*d)

	if (l/d < 50) {
		document.write('L/d < 50.<br>')
	} else {
		document.write(`l = ${l}<br>`)
	}
}